diff --git a/apps/api/migrations/0013_flash_sales.sql b/apps/api/migrations/0013_flash_sales.sql
new file mode 100644
index 0000000..42384e7
--- /dev/null
+++ b/apps/api/migrations/0013_flash_sales.sql
@@ -0,0 +1,42 @@
+-- Flash sales: a shop-owned timed session with SKU-level activity items that
+-- hold their own reserved inventory and per-customer limit. An order line that
+-- received activity pricing points at the item it consumed, so cancellation can
+-- restore both the SKU stock and the reserved activity stock.
+
+CREATE TABLE flash_sale_sessions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    shop_id UUID NOT NULL REFERENCES shops (id) ON DELETE CASCADE,
+    label JSONB NOT NULL,
+    starts_at TIMESTAMPTZ NOT NULL,
+    ends_at TIMESTAMPTZ NOT NULL,
+    enabled BOOLEAN NOT NULL DEFAULT TRUE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    CONSTRAINT flash_sale_sessions_window CHECK (ends_at >= starts_at)
+);
+
+CREATE INDEX flash_sale_sessions_shop_idx ON flash_sale_sessions (shop_id);
+
+CREATE TABLE flash_sale_items (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    session_id UUID NOT NULL REFERENCES flash_sale_sessions (id) ON DELETE CASCADE,
+    sku_id UUID NOT NULL REFERENCES skus (id) ON DELETE CASCADE,
+    sale_price_minor BIGINT NOT NULL CHECK (sale_price_minor > 0),
+    currency CHAR(3) NOT NULL REFERENCES currencies (code),
+    -- Remaining activity inventory; decremented conditionally at checkout.
+    reserved_stock INT NOT NULL CHECK (reserved_stock >= 0),
+    sold_count INT NOT NULL DEFAULT 0 CHECK (sold_count >= 0),
+    per_customer_limit INT NOT NULL CHECK (per_customer_limit > 0),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE UNIQUE INDEX flash_sale_items_session_sku_idx ON flash_sale_items (session_id, sku_id);
+CREATE INDEX flash_sale_items_sku_idx ON flash_sale_items (sku_id);
+
+-- Nullable: standard-priced lines keep NULL, and deleting an activity item
+-- leaves the order line intact.
+ALTER TABLE order_items
+    ADD COLUMN flash_sale_item_id UUID REFERENCES flash_sale_items (id) ON DELETE SET NULL;
+
+CREATE INDEX idx_order_items_flash ON order_items (flash_sale_item_id);
diff --git a/apps/api/src/models.rs b/apps/api/src/models.rs
index a127890..a02889d 100644
--- a/apps/api/src/models.rs
+++ b/apps/api/src/models.rs
@@ -208,6 +208,8 @@ pub struct OrderItem {
     pub image: Option<String>,
     pub unit_price_minor: i64,
     pub qty: i32,
+    /// Set when this line received flash-sale pricing.
+    pub flash_sale_item_id: Option<Uuid>,
 }
 
 #[derive(Debug, Clone, Serialize, sqlx::FromRow)]
@@ -402,6 +404,40 @@ pub struct IntegralOrderItem {
 pub const INTEGRAL_ORDER_ITEM_COLUMNS: &str =
     "id, order_id, product_id, name, image, points_price, qty";
 
+/// Shop-owned timed flash-sale session.
+#[derive(Debug, Clone, Serialize, sqlx::FromRow)]
+pub struct FlashSaleSession {
+    pub id: Uuid,
+    pub shop_id: Uuid,
+    pub label: serde_json::Value,
+    pub starts_at: DateTime<Utc>,
+    pub ends_at: DateTime<Utc>,
+    pub enabled: bool,
+    pub created_at: DateTime<Utc>,
+    pub updated_at: DateTime<Utc>,
+}
+
+pub const FLASH_SALE_SESSION_COLUMNS: &str =
+    "id, shop_id, label, starts_at, ends_at, enabled, created_at, updated_at";
+
+/// SKU-level activity item with its own reserved inventory and customer limit.
+#[derive(Debug, Clone, Serialize, sqlx::FromRow)]
+pub struct FlashSaleItem {
+    pub id: Uuid,
+    pub session_id: Uuid,
+    pub sku_id: Uuid,
+    pub sale_price_minor: i64,
+    pub currency: String,
+    pub reserved_stock: i32,
+    pub sold_count: i32,
+    pub per_customer_limit: i32,
+    pub created_at: DateTime<Utc>,
+    pub updated_at: DateTime<Utc>,
+}
+
+pub const FLASH_SALE_ITEM_COLUMNS: &str = "id, session_id, sku_id, sale_price_minor, currency, \
+     reserved_stock, sold_count, per_customer_limit, created_at, updated_at";
+
 #[derive(Debug, Clone, Serialize, sqlx::FromRow)]
 pub struct AddressBookEntry {
     pub id: Uuid,
diff --git a/apps/api/src/modules/flash_sale/dto.rs b/apps/api/src/modules/flash_sale/dto.rs
new file mode 100644
index 0000000..c39cbfc
--- /dev/null
+++ b/apps/api/src/modules/flash_sale/dto.rs
@@ -0,0 +1,86 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+use crate::models::FlashSaleSession;
+
+#[derive(Deserialize)]
+pub struct SessionInput {
+    pub label: serde_json::Value,
+    pub starts_at: DateTime<Utc>,
+    pub ends_at: DateTime<Utc>,
+    #[serde(default = "default_enabled")]
+    pub enabled: bool,
+}
+
+#[derive(Deserialize)]
+pub struct ItemInput {
+    pub sku_id: Uuid,
+    pub sale_price_minor: i64,
+    pub currency: String,
+    pub reserved_stock: i32,
+    pub per_customer_limit: i32,
+}
+
+fn default_enabled() -> bool {
+    true
+}
+
+/// Shop-side item joined with the SKU and catalog price it discounts.
+#[derive(Debug, Serialize, sqlx::FromRow)]
+pub struct ShopItemView {
+    pub id: Uuid,
+    pub session_id: Uuid,
+    pub sku_id: Uuid,
+    pub sku_code: String,
+    pub product_name: serde_json::Value,
+    pub sale_price_minor: i64,
+    pub currency: String,
+    pub original_price_minor: i64,
+    pub original_currency: String,
+    pub reserved_stock: i32,
+    pub sold_count: i32,
+    pub per_customer_limit: i32,
+}
+
+#[derive(Debug, Serialize)]
+pub struct ShopSessionView {
+    #[serde(flatten)]
+    pub session: FlashSaleSession,
+    pub items: Vec<ShopItemView>,
+}
+
+/// Public discovery item: everything a shopper needs to add it to the cart.
+#[derive(Debug, Serialize, sqlx::FromRow)]
+pub struct PublicItemView {
+    pub id: Uuid,
+    pub session_id: Uuid,
+    pub sku_id: Uuid,
+    pub product_id: Uuid,
+    pub product_slug: String,
+    pub product_name: serde_json::Value,
+    pub image: Option<String>,
+    pub sku_code: String,
+    pub sale_price_minor: i64,
+    pub currency: String,
+    pub original_price_minor: i64,
+    pub original_currency: String,
+    pub reserved_stock: i32,
+    pub sold_count: i32,
+    pub per_customer_limit: i32,
+}
+
+#[derive(Debug, Serialize)]
+pub struct PublicSessionView {
+    #[serde(flatten)]
+    pub session: FlashSaleSession,
+    pub items: Vec<PublicItemView>,
+}
+
+/// The part of a cart line that received activity pricing.
+#[derive(Debug, Clone, Copy)]
+pub struct LineActivity {
+    pub item_id: Uuid,
+    pub unit_price_minor: i64,
+    pub qty: i32,
+}
diff --git a/apps/api/src/modules/flash_sale/handlers.rs b/apps/api/src/modules/flash_sale/handlers.rs
new file mode 100644
index 0000000..02c055d
--- /dev/null
+++ b/apps/api/src/modules/flash_sale/handlers.rs
@@ -0,0 +1,114 @@
+use axum::{
+    extract::{Path, State},
+    http::StatusCode,
+    routing::{get, post, put},
+    Json, Router,
+};
+use uuid::Uuid;
+
+use crate::auth::AuthUser;
+use crate::error::ApiResult;
+use crate::models::{FlashSaleItem, FlashSaleSession};
+use crate::state::AppState;
+
+use super::dto::{ItemInput, PublicSessionView, SessionInput, ShopSessionView};
+use super::service;
+
+pub fn router() -> Router<AppState> {
+    Router::new()
+        // Public: active sessions and their purchasable items.
+        .route("/flash-sales", get(public_active))
+        .route("/shop/flash-sales", get(shop_list).post(shop_create))
+        .route(
+            "/shop/flash-sales/{id}",
+            put(shop_update).delete(shop_delete),
+        )
+        .route("/shop/flash-sales/{id}/items", post(shop_add_item))
+        .route(
+            "/shop/flash-sale-items/{id}",
+            put(shop_update_item).delete(shop_delete_item),
+        )
+}
+
+async fn public_active(
+    State(state): State<AppState>,
+) -> ApiResult<Json<Vec<PublicSessionView>>> {
+    Ok(Json(service::public_active(&state).await?))
+}
+
+async fn shop_list(
+    State(state): State<AppState>,
+    auth: AuthUser,
+) -> ApiResult<Json<Vec<ShopSessionView>>> {
+    let shop_id = auth.require_shop()?;
+    Ok(Json(service::list_for_shop(&state, shop_id).await?))
+}
+
+async fn shop_create(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Json(body): Json<SessionInput>,
+) -> ApiResult<(StatusCode, Json<FlashSaleSession>)> {
+    let shop_id = auth.require_shop()?;
+    Ok((
+        StatusCode::CREATED,
+        Json(service::create_session(&state, shop_id, body).await?),
+    ))
+}
+
+async fn shop_update(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<Uuid>,
+    Json(body): Json<SessionInput>,
+) -> ApiResult<Json<FlashSaleSession>> {
+    let shop_id = auth.require_shop()?;
+    Ok(Json(
+        service::update_session(&state, shop_id, id, body).await?,
+    ))
+}
+
+async fn shop_delete(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<Uuid>,
+) -> ApiResult<StatusCode> {
+    let shop_id = auth.require_shop()?;
+    service::delete_session(&state, shop_id, id).await?;
+    Ok(StatusCode::NO_CONTENT)
+}
+
+async fn shop_add_item(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<Uuid>,
+    Json(body): Json<ItemInput>,
+) -> ApiResult<(StatusCode, Json<FlashSaleItem>)> {
+    let shop_id = auth.require_shop()?;
+    Ok((
+        StatusCode::CREATED,
+        Json(service::add_item(&state, shop_id, id, body).await?),
+    ))
+}
+
+async fn shop_update_item(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<Uuid>,
+    Json(body): Json<ItemInput>,
+) -> ApiResult<Json<FlashSaleItem>> {
+    let shop_id = auth.require_shop()?;
+    Ok(Json(
+        service::update_item(&state, shop_id, id, body).await?,
+    ))
+}
+
+async fn shop_delete_item(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<Uuid>,
+) -> ApiResult<StatusCode> {
+    let shop_id = auth.require_shop()?;
+    service::delete_item(&state, shop_id, id).await?;
+    Ok(StatusCode::NO_CONTENT)
+}
diff --git a/apps/api/src/modules/flash_sale/mod.rs b/apps/api/src/modules/flash_sale/mod.rs
new file mode 100644
index 0000000..2cdf8fe
--- /dev/null
+++ b/apps/api/src/modules/flash_sale/mod.rs
@@ -0,0 +1,12 @@
+mod dto;
+mod handlers;
+mod repo;
+pub mod service;
+
+use axum::Router;
+
+use crate::state::AppState;
+
+pub fn router() -> Router<AppState> {
+    handlers::router()
+}
diff --git a/apps/api/src/modules/flash_sale/repo.rs b/apps/api/src/modules/flash_sale/repo.rs
new file mode 100644
index 0000000..8f3a31a
--- /dev/null
+++ b/apps/api/src/modules/flash_sale/repo.rs
@@ -0,0 +1,409 @@
+use sqlx::{PgConnection, PgExecutor};
+use uuid::Uuid;
+
+use crate::error::{ApiError, ApiResult};
+use crate::models::{
+    FlashSaleItem, FlashSaleSession, FLASH_SALE_ITEM_COLUMNS, FLASH_SALE_SESSION_COLUMNS,
+};
+
+use super::dto::{ItemInput, PublicItemView, SessionInput, ShopItemView};
+
+/// Qualified item columns for queries that join `flash_sale_sessions`, whose
+/// own `id` would otherwise make bare column names ambiguous.
+const ITEM_COLS_Q: &str = "i.id, i.session_id, i.sku_id, i.sale_price_minor, i.currency, \
+     i.reserved_stock, i.sold_count, i.per_customer_limit, i.created_at, i.updated_at";
+
+// ---- sessions ----
+
+pub async fn currency_enabled<'e, E: PgExecutor<'e>>(exec: E, code: &str) -> ApiResult<bool> {
+    Ok(sqlx::query_scalar(
+        "SELECT EXISTS(SELECT 1 FROM currencies WHERE code = $1 AND enabled = TRUE)",
+    )
+    .bind(code)
+    .fetch_one(exec)
+    .await?)
+}
+
+pub async fn list_sessions_for_shop<'e, E: PgExecutor<'e>>(
+    exec: E,
+    shop_id: Uuid,
+) -> ApiResult<Vec<FlashSaleSession>> {
+    Ok(sqlx::query_as::<_, FlashSaleSession>(&format!(
+        "SELECT {FLASH_SALE_SESSION_COLUMNS} FROM flash_sale_sessions
+          WHERE shop_id = $1 ORDER BY starts_at DESC"
+    ))
+    .bind(shop_id)
+    .fetch_all(exec)
+    .await?)
+}
+
+pub async fn get_session_own<'e, E: PgExecutor<'e>>(
+    exec: E,
+    shop_id: Uuid,
+    id: Uuid,
+) -> ApiResult<FlashSaleSession> {
+    sqlx::query_as::<_, FlashSaleSession>(&format!(
+        "SELECT {FLASH_SALE_SESSION_COLUMNS} FROM flash_sale_sessions
+          WHERE id = $1 AND shop_id = $2"
+    ))
+    .bind(id)
+    .bind(shop_id)
+    .fetch_optional(exec)
+    .await?
+    .ok_or_else(|| ApiError::NotFound("flash-sale session".into()))
+}
+
+pub async fn insert_session(
+    tx: &mut PgConnection,
+    shop_id: Uuid,
+    body: &SessionInput,
+) -> ApiResult<FlashSaleSession> {
+    Ok(sqlx::query_as::<_, FlashSaleSession>(&format!(
+        "INSERT INTO flash_sale_sessions (shop_id, label, starts_at, ends_at, enabled)
+         VALUES ($1, $2, $3, $4, $5)
+         RETURNING {FLASH_SALE_SESSION_COLUMNS}"
+    ))
+    .bind(shop_id)
+    .bind(&body.label)
+    .bind(body.starts_at)
+    .bind(body.ends_at)
+    .bind(body.enabled)
+    .fetch_one(&mut *tx)
+    .await?)
+}
+
+pub async fn update_session(
+    tx: &mut PgConnection,
+    shop_id: Uuid,
+    id: Uuid,
+    body: &SessionInput,
+) -> ApiResult<FlashSaleSession> {
+    sqlx::query_as::<_, FlashSaleSession>(&format!(
+        "UPDATE flash_sale_sessions
+            SET label = $3, starts_at = $4, ends_at = $5, enabled = $6, updated_at = now()
+          WHERE id = $1 AND shop_id = $2
+         RETURNING {FLASH_SALE_SESSION_COLUMNS}"
+    ))
+    .bind(id)
+    .bind(shop_id)
+    .bind(&body.label)
+    .bind(body.starts_at)
+    .bind(body.ends_at)
+    .bind(body.enabled)
+    .fetch_optional(&mut *tx)
+    .await?
+    .ok_or_else(|| ApiError::NotFound("flash-sale session".into()))
+}
+
+pub async fn delete_session(tx: &mut PgConnection, shop_id: Uuid, id: Uuid) -> ApiResult<()> {
+    let result = sqlx::query("DELETE FROM flash_sale_sessions WHERE id = $1 AND shop_id = $2")
+        .bind(id)
+        .bind(shop_id)
+        .execute(&mut *tx)
+        .await?;
+    if result.rows_affected() == 0 {
+        return Err(ApiError::NotFound("flash-sale session".into()));
+    }
+    Ok(())
+}
+
+// ---- activity items ----
+
+pub async fn list_items_for_shop<'e, E: PgExecutor<'e>>(
+    exec: E,
+    shop_id: Uuid,
+) -> ApiResult<Vec<ShopItemView>> {
+    Ok(sqlx::query_as::<_, ShopItemView>(
+        "SELECT i.id, i.session_id, i.sku_id, s.sku_code, p.name AS product_name,
+                i.sale_price_minor, i.currency,
+                s.price_minor AS original_price_minor, s.currency AS original_currency,
+                i.reserved_stock, i.sold_count, i.per_customer_limit
+           FROM flash_sale_items i
+           JOIN flash_sale_sessions fs ON fs.id = i.session_id
+           JOIN skus s ON s.id = i.sku_id
+           JOIN products p ON p.id = s.product_id
+          WHERE fs.shop_id = $1
+          ORDER BY i.created_at",
+    )
+    .bind(shop_id)
+    .fetch_all(exec)
+    .await?)
+}
+
+pub async fn get_item_own<'e, E: PgExecutor<'e>>(
+    exec: E,
+    shop_id: Uuid,
+    id: Uuid,
+) -> ApiResult<FlashSaleItem> {
+    sqlx::query_as::<_, FlashSaleItem>(&format!(
+        "SELECT {ITEM_COLS_Q} FROM flash_sale_items i
+           JOIN flash_sale_sessions s ON s.id = i.session_id
+          WHERE i.id = $1 AND s.shop_id = $2"
+    ))
+    .bind(id)
+    .bind(shop_id)
+    .fetch_optional(exec)
+    .await?
+    .ok_or_else(|| ApiError::NotFound("flash-sale item".into()))
+}
+
+pub async fn insert_item(
+    tx: &mut PgConnection,
+    session_id: Uuid,
+    body: &ItemInput,
+) -> Result<FlashSaleItem, sqlx::Error> {
+    sqlx::query_as::<_, FlashSaleItem>(&format!(
+        "INSERT INTO flash_sale_items
+             (session_id, sku_id, sale_price_minor, currency, reserved_stock, per_customer_limit)
+         VALUES ($1, $2, $3, $4, $5, $6)
+         RETURNING {FLASH_SALE_ITEM_COLUMNS}"
+    ))
+    .bind(session_id)
+    .bind(body.sku_id)
+    .bind(body.sale_price_minor)
+    .bind(body.currency.to_uppercase())
+    .bind(body.reserved_stock)
+    .bind(body.per_customer_limit)
+    .fetch_one(&mut *tx)
+    .await
+}
+
+pub async fn update_item(
+    tx: &mut PgConnection,
+    shop_id: Uuid,
+    id: Uuid,
+    body: &ItemInput,
+) -> ApiResult<FlashSaleItem> {
+    sqlx::query_as::<_, FlashSaleItem>(
+        "UPDATE flash_sale_items i
+            SET sku_id = $3, sale_price_minor = $4, currency = $5,
+                reserved_stock = $6, per_customer_limit = $7, updated_at = now()
+           FROM flash_sale_sessions s
+          WHERE i.id = $1 AND i.session_id = s.id AND s.shop_id = $2
+         RETURNING i.id, i.session_id, i.sku_id, i.sale_price_minor, i.currency,
+                   i.reserved_stock, i.sold_count, i.per_customer_limit, i.created_at, i.updated_at",
+    )
+    .bind(id)
+    .bind(shop_id)
+    .bind(body.sku_id)
+    .bind(body.sale_price_minor)
+    .bind(body.currency.to_uppercase())
+    .bind(body.reserved_stock)
+    .bind(body.per_customer_limit)
+    .fetch_optional(&mut *tx)
+    .await?
+    .ok_or_else(|| ApiError::NotFound("flash-sale item".into()))
+}
+
+pub async fn delete_item(tx: &mut PgConnection, shop_id: Uuid, id: Uuid) -> ApiResult<()> {
+    let result = sqlx::query(
+        "DELETE FROM flash_sale_items i
+          USING flash_sale_sessions s
+          WHERE i.id = $1 AND i.session_id = s.id AND s.shop_id = $2",
+    )
+    .bind(id)
+    .bind(shop_id)
+    .execute(&mut *tx)
+    .await?;
+    if result.rows_affected() == 0 {
+        return Err(ApiError::NotFound("flash-sale item".into()));
+    }
+    Ok(())
+}
+
+/// The SKU must be active, published, and owned by the session's shop.
+pub async fn sku_is_own_active<'e, E: PgExecutor<'e>>(
+    exec: E,
+    shop_id: Uuid,
+    sku_id: Uuid,
+) -> ApiResult<bool> {
+    Ok(sqlx::query_scalar(
+        "SELECT EXISTS(
+             SELECT 1 FROM skus s
+             JOIN products p ON p.id = s.product_id
+             WHERE s.id = $1 AND p.shop_id = $2
+               AND s.active = TRUE AND p.status = 'published'
+         )",
+    )
+    .bind(sku_id)
+    .bind(shop_id)
+    .fetch_one(exec)
+    .await?)
+}
+
+/// Another enabled session whose window overlaps and already lists this SKU.
+pub async fn overlapping_sale_exists<'e, E: PgExecutor<'e>>(
+    exec: E,
+    sku_id: Uuid,
+    session_id: Uuid,
+    starts_at: chrono::DateTime<chrono::Utc>,
+    ends_at: chrono::DateTime<chrono::Utc>,
+) -> ApiResult<bool> {
+    Ok(sqlx::query_scalar(
+        "SELECT EXISTS(
+             SELECT 1 FROM flash_sale_items i
+             JOIN flash_sale_sessions s ON s.id = i.session_id
+             WHERE i.sku_id = $1 AND s.id <> $2 AND s.enabled = TRUE
+               AND s.starts_at <= $4 AND s.ends_at >= $3
+         )",
+    )
+    .bind(sku_id)
+    .bind(session_id)
+    .bind(starts_at)
+    .bind(ends_at)
+    .fetch_one(exec)
+    .await?)
+}
+
+/// The group-buying capability lands after this one, so its overlap check is
+/// dormant until its table exists.
+pub async fn group_buying_table_exists<'e, E: PgExecutor<'e>>(exec: E) -> ApiResult<bool> {
+    Ok(sqlx::query_scalar("SELECT to_regclass('public.group_buying_activities') IS NOT NULL")
+        .fetch_one(exec)
+        .await?)
+}
+
+pub async fn overlapping_group_buying_exists(
+    tx: &mut PgConnection,
+    sku_id: Uuid,
+    starts_at: chrono::DateTime<chrono::Utc>,
+    ends_at: chrono::DateTime<chrono::Utc>,
+) -> ApiResult<bool> {
+    Ok(sqlx::query_scalar(
+        "SELECT EXISTS(
+             SELECT 1 FROM group_buying_activities a
+             WHERE a.sku_id = $1 AND a.enabled = TRUE
+               AND a.starts_at <= $3 AND a.ends_at >= $2
+         )",
+    )
+    .bind(sku_id)
+    .bind(starts_at)
+    .bind(ends_at)
+    .fetch_one(&mut *tx)
+    .await?)
+}
+
+// ---- public discovery ----
+
+pub async fn list_active_sessions<'e, E: PgExecutor<'e>>(
+    exec: E,
+) -> ApiResult<Vec<FlashSaleSession>> {
+    Ok(sqlx::query_as::<_, FlashSaleSession>(&format!(
+        "SELECT {FLASH_SALE_SESSION_COLUMNS} FROM flash_sale_sessions
+          WHERE enabled = TRUE AND now() BETWEEN starts_at AND ends_at
+          ORDER BY starts_at"
+    ))
+    .fetch_all(exec)
+    .await?)
+}
+
+pub async fn list_public_items<'e, E: PgExecutor<'e>>(
+    exec: E,
+    session_ids: &[Uuid],
+) -> ApiResult<Vec<PublicItemView>> {
+    Ok(sqlx::query_as::<_, PublicItemView>(
+        "SELECT i.id, i.session_id, i.sku_id, s.product_id, p.slug AS product_slug,
+                p.name AS product_name, (p.images ->> 0) AS image, s.sku_code,
+                i.sale_price_minor, i.currency,
+                s.price_minor AS original_price_minor, s.currency AS original_currency,
+                i.reserved_stock, i.sold_count, i.per_customer_limit
+           FROM flash_sale_items i
+           JOIN skus s ON s.id = i.sku_id
+           JOIN products p ON p.id = s.product_id
+          WHERE i.session_id = ANY($1)
+            AND s.active = TRUE AND p.status = 'published'
+          ORDER BY i.created_at",
+    )
+    .bind(session_ids)
+    .fetch_all(exec)
+    .await?)
+}
+
+// ---- checkout ----
+
+/// Activity item ids that could apply to these cart SKUs right now.
+pub async fn candidate_item_ids<'e, E: PgExecutor<'e>>(
+    exec: E,
+    sku_ids: &[Uuid],
+) -> ApiResult<Vec<Uuid>> {
+    Ok(sqlx::query_scalar(
+        "SELECT i.id FROM flash_sale_items i
+           JOIN flash_sale_sessions s ON s.id = i.session_id
+          WHERE i.sku_id = ANY($1) AND s.enabled = TRUE
+            AND now() BETWEEN s.starts_at AND s.ends_at",
+    )
+    .bind(sku_ids)
+    .fetch_all(exec)
+    .await?)
+}
+
+/// Lock candidates in primary-key order and re-prove they are still active.
+pub async fn lock_active_items(
+    tx: &mut PgConnection,
+    ids: &[Uuid],
+) -> ApiResult<Vec<FlashSaleItem>> {
+    Ok(sqlx::query_as::<_, FlashSaleItem>(&format!(
+        "SELECT {ITEM_COLS_Q} FROM flash_sale_items i
+           JOIN flash_sale_sessions s ON s.id = i.session_id
+          WHERE i.id = ANY($1) AND s.enabled = TRUE
+            AND now() BETWEEN s.starts_at AND s.ends_at
+          ORDER BY i.id
+          FOR UPDATE OF i"
+    ))
+    .bind(ids)
+    .fetch_all(&mut *tx)
+    .await?)
+}
+
+/// Activity units this customer already holds on non-cancelled orders.
+pub async fn purchased_qty(
+    tx: &mut PgConnection,
+    user_id: Uuid,
+    item_id: Uuid,
+) -> ApiResult<i64> {
+    Ok(sqlx::query_scalar(
+        "SELECT COALESCE(SUM(oi.qty), 0)
+           FROM order_items oi
+           JOIN orders o ON o.id = oi.order_id
+          WHERE oi.flash_sale_item_id = $1 AND o.user_id = $2
+            AND o.status <> 'cancelled'",
+    )
+    .bind(item_id)
+    .bind(user_id)
+    .fetch_one(&mut *tx)
+    .await?)
+}
+
+/// Guarded reserved-stock decrement; `sold_count` tracks sell-through.
+pub async fn decrement_reserved(tx: &mut PgConnection, id: Uuid, qty: i32) -> ApiResult<()> {
+    let result = sqlx::query(
+        "UPDATE flash_sale_items
+            SET reserved_stock = reserved_stock - $2, sold_count = sold_count + $2,
+                updated_at = now()
+          WHERE id = $1 AND reserved_stock >= $2",
+    )
+    .bind(id)
+    .bind(qty)
+    .execute(&mut *tx)
+    .await?;
+    if result.rows_affected() == 0 {
+        return Err(ApiError::Conflict("insufficient flash-sale stock".into()));
+    }
+    Ok(())
+}
+
+/// Pending-payment cancellation returns reserved activity stock.
+pub async fn restore_for_order(tx: &mut PgConnection, order_id: Uuid) -> ApiResult<()> {
+    sqlx::query(
+        "UPDATE flash_sale_items i
+            SET reserved_stock = i.reserved_stock + oi.qty,
+                sold_count = GREATEST(i.sold_count - oi.qty, 0),
+                updated_at = now()
+           FROM order_items oi
+          WHERE oi.order_id = $1 AND oi.flash_sale_item_id = i.id",
+    )
+    .bind(order_id)
+    .execute(&mut *tx)
+    .await?;
+    Ok(())
+}
diff --git a/apps/api/src/modules/flash_sale/service.rs b/apps/api/src/modules/flash_sale/service.rs
new file mode 100644
index 0000000..e5e4b21
--- /dev/null
+++ b/apps/api/src/modules/flash_sale/service.rs
@@ -0,0 +1,283 @@
+use std::collections::HashMap;
+
+use serde_json::Value;
+use sqlx::PgConnection;
+use uuid::Uuid;
+
+use crate::error::{unique_conflict, ApiError, ApiResult};
+use crate::models::{Currency, FlashSaleItem, FlashSaleSession};
+use crate::money::convert_minor;
+use crate::state::AppState;
+
+use super::dto::{
+    ItemInput, LineActivity, PublicSessionView, SessionInput, ShopItemView, ShopSessionView,
+};
+use super::repo;
+
+// ---- shop management ----
+
+pub async fn list_for_shop(state: &AppState, shop_id: Uuid) -> ApiResult<Vec<ShopSessionView>> {
+    let sessions = repo::list_sessions_for_shop(&state.db, shop_id).await?;
+    let items = repo::list_items_for_shop(&state.db, shop_id).await?;
+    let mut by_session: HashMap<Uuid, Vec<ShopItemView>> = HashMap::new();
+    for item in items {
+        by_session.entry(item.session_id).or_default().push(item);
+    }
+    Ok(sessions
+        .into_iter()
+        .map(|session| ShopSessionView {
+            items: by_session.remove(&session.id).unwrap_or_default(),
+            session,
+        })
+        .collect())
+}
+
+pub async fn create_session(
+    state: &AppState,
+    shop_id: Uuid,
+    body: SessionInput,
+) -> ApiResult<FlashSaleSession> {
+    validate_session(&body)?;
+    let mut tx = state.db.begin().await?;
+    let session = repo::insert_session(&mut tx, shop_id, &body).await?;
+    tx.commit().await?;
+    Ok(session)
+}
+
+pub async fn update_session(
+    state: &AppState,
+    shop_id: Uuid,
+    id: Uuid,
+    body: SessionInput,
+) -> ApiResult<FlashSaleSession> {
+    validate_session(&body)?;
+    let mut tx = state.db.begin().await?;
+    let session = repo::update_session(&mut tx, shop_id, id, &body).await?;
+    tx.commit().await?;
+    Ok(session)
+}
+
+pub async fn delete_session(state: &AppState, shop_id: Uuid, id: Uuid) -> ApiResult<()> {
+    let mut tx = state.db.begin().await?;
+    repo::delete_session(&mut tx, shop_id, id).await?;
+    tx.commit().await?;
+    Ok(())
+}
+
+pub async fn add_item(
+    state: &AppState,
+    shop_id: Uuid,
+    session_id: Uuid,
+    body: ItemInput,
+) -> ApiResult<FlashSaleItem> {
+    validate_item(&body)?;
+    let mut tx = state.db.begin().await?;
+    ensure_currency(&mut tx, &body.currency).await?;
+    let session = repo::get_session_own(&mut *tx, shop_id, session_id).await?;
+    ensure_activity_slot(&mut tx, shop_id, &session, body.sku_id).await?;
+    let item = repo::insert_item(&mut tx, session_id, &body)
+        .await
+        .map_err(|e| unique_conflict(e, "SKU is already in this session"))?;
+    tx.commit().await?;
+    Ok(item)
+}
+
+pub async fn update_item(
+    state: &AppState,
+    shop_id: Uuid,
+    id: Uuid,
+    body: ItemInput,
+) -> ApiResult<FlashSaleItem> {
+    validate_item(&body)?;
+    let mut tx = state.db.begin().await?;
+    ensure_currency(&mut tx, &body.currency).await?;
+    let existing = repo::get_item_own(&mut *tx, shop_id, id).await?;
+    let session = repo::get_session_own(&mut *tx, shop_id, existing.session_id).await?;
+    ensure_activity_slot(&mut tx, shop_id, &session, body.sku_id).await?;
+    let item = repo::update_item(&mut tx, shop_id, id, &body).await?;
+    tx.commit().await?;
+    Ok(item)
+}
+
+pub async fn delete_item(state: &AppState, shop_id: Uuid, id: Uuid) -> ApiResult<()> {
+    let mut tx = state.db.begin().await?;
+    repo::delete_item(&mut tx, shop_id, id).await?;
+    tx.commit().await?;
+    Ok(())
+}
+
+// ---- public discovery ----
+
+pub async fn public_active(state: &AppState) -> ApiResult<Vec<PublicSessionView>> {
+    let sessions = repo::list_active_sessions(&state.db).await?;
+    let ids: Vec<Uuid> = sessions.iter().map(|s| s.id).collect();
+    let items = if ids.is_empty() {
+        Vec::new()
+    } else {
+        repo::list_public_items(&state.db, &ids).await?
+    };
+    let mut by_session: HashMap<Uuid, Vec<_>> = HashMap::new();
+    for item in items {
+        by_session.entry(item.session_id).or_default().push(item);
+    }
+    Ok(sessions
+        .into_iter()
+        .map(|session| PublicSessionView {
+            items: by_session.remove(&session.id).unwrap_or_default(),
+            session,
+        })
+        .collect())
+}
+
+// ---- checkout composition ----
+
+/// Resolve the activity-priced part of each cart line server-side. Candidates
+/// are locked in primary-key order, so concurrent checkouts cannot oversell the
+/// reserved stock; the customer's remaining allowance caps the activity units.
+pub async fn resolve_activities(
+    tx: &mut PgConnection,
+    user_id: Uuid,
+    lines: &[(Uuid, i32)],
+    target: &Currency,
+    currencies: &[Currency],
+) -> ApiResult<HashMap<Uuid, LineActivity>> {
+    let sku_ids: Vec<Uuid> = lines.iter().map(|(sku_id, _)| *sku_id).collect();
+    let candidate_ids = repo::candidate_item_ids(&mut *tx, &sku_ids).await?;
+    if candidate_ids.is_empty() {
+        return Ok(HashMap::new());
+    }
+    let locked = repo::lock_active_items(tx, &candidate_ids).await?;
+    let mut by_sku: HashMap<Uuid, FlashSaleItem> = HashMap::new();
+    for item in locked {
+        by_sku.entry(item.sku_id).or_insert(item);
+    }
+
+    let mut resolved = HashMap::new();
+    for (sku_id, qty) in lines {
+        let Some(item) = by_sku.get(sku_id) else {
+            continue;
+        };
+        let bought = repo::purchased_qty(&mut *tx, user_id, item.id).await?;
+        let allowance = item.per_customer_limit as i64 - bought;
+        let activity_qty = (*qty as i64).min(allowance).min(item.reserved_stock as i64);
+        if activity_qty <= 0 {
+            continue;
+        }
+        let from = currencies
+            .iter()
+            .find(|c| c.code == item.currency)
+            .ok_or_else(|| {
+                ApiError::BadRequest(format!("currency {} is disabled", item.currency))
+            })?;
+        let unit_price_minor = convert_minor(item.sale_price_minor, from, target)?;
+        resolved.insert(
+            *sku_id,
+            LineActivity {
+                item_id: item.id,
+                unit_price_minor,
+                qty: activity_qty as i32,
+            },
+        );
+    }
+    Ok(resolved)
+}
+
+/// Guarded reserved-stock decrement for the activity units of a line.
+pub async fn consume(tx: &mut PgConnection, item_id: Uuid, qty: i32) -> ApiResult<()> {
+    repo::decrement_reserved(tx, item_id, qty).await
+}
+
+/// Pending-payment cancellation returns reserved activity stock.
+pub async fn restore_for_order(tx: &mut PgConnection, order_id: Uuid) -> ApiResult<()> {
+    repo::restore_for_order(tx, order_id).await
+}
+
+// ---- validation ----
+
+async fn ensure_activity_slot(
+    tx: &mut PgConnection,
+    shop_id: Uuid,
+    session: &FlashSaleSession,
+    sku_id: Uuid,
+) -> ApiResult<()> {
+    if !repo::sku_is_own_active(&mut *tx, shop_id, sku_id).await? {
+        return Err(ApiError::BadRequest(
+            "SKU is not an active published product of this shop".into(),
+        ));
+    }
+    if repo::overlapping_sale_exists(
+        &mut *tx,
+        sku_id,
+        session.id,
+        session.starts_at,
+        session.ends_at,
+    )
+    .await?
+    {
+        return Err(ApiError::Conflict(
+            "SKU already has an overlapping flash sale".into(),
+        ));
+    }
+    if repo::group_buying_table_exists(&mut *tx).await?
+        && repo::overlapping_group_buying_exists(tx, sku_id, session.starts_at, session.ends_at)
+            .await?
+    {
+        return Err(ApiError::Conflict(
+            "SKU already has an overlapping group-buying activity".into(),
+        ));
+    }
+    Ok(())
+}
+
+async fn ensure_currency(tx: &mut PgConnection, code: &str) -> ApiResult<()> {
+    if !repo::currency_enabled(&mut *tx, &code.to_uppercase()).await? {
+        return Err(ApiError::BadRequest(
+            "currency is unknown or disabled".into(),
+        ));
+    }
+    Ok(())
+}
+
+fn validate_session(body: &SessionInput) -> ApiResult<()> {
+    bilingual(&body.label, "label")?;
+    if body.ends_at < body.starts_at {
+        return Err(ApiError::BadRequest(
+            "ends_at must not precede starts_at".into(),
+        ));
+    }
+    Ok(())
+}
+
+fn validate_item(body: &ItemInput) -> ApiResult<()> {
+    if body.sale_price_minor <= 0 {
+        return Err(ApiError::BadRequest(
+            "sale_price_minor must be positive".into(),
+        ));
+    }
+    if body.reserved_stock < 0 {
+        return Err(ApiError::BadRequest(
+            "reserved_stock must not be negative".into(),
+        ));
+    }
+    if body.per_customer_limit <= 0 {
+        return Err(ApiError::BadRequest(
+            "per_customer_limit must be positive".into(),
+        ));
+    }
+    Ok(())
+}
+
+fn bilingual(label: &Value, field: &str) -> ApiResult<()> {
+    let ok = ["en", "zh"].iter().all(|code| {
+        label
+            .get(code)
+            .and_then(Value::as_str)
+            .is_some_and(|s| !s.trim().is_empty())
+    });
+    if !ok {
+        return Err(ApiError::BadRequest(format!(
+            "{field} needs non-empty en and zh"
+        )));
+    }
+    Ok(())
+}
diff --git a/apps/api/src/modules/mod.rs b/apps/api/src/modules/mod.rs
index aa8fc44..e037004 100644
--- a/apps/api/src/modules/mod.rs
+++ b/apps/api/src/modules/mod.rs
@@ -6,6 +6,7 @@ pub mod catalog;
 pub mod content;
 pub mod coupon;
 pub mod currency;
+pub mod flash_sale;
 pub mod fulfillment;
 pub mod health;
 pub mod identity;
@@ -28,6 +29,7 @@ pub fn api_router() -> Router<AppState> {
         .merge(content::router())
         .merge(cart::router())
         .merge(coupon::router())
+        .merge(flash_sale::router())
         .merge(order::router())
         .merge(points::router())
         .merge(shop::router())
diff --git a/apps/api/src/modules/order/repo.rs b/apps/api/src/modules/order/repo.rs
index d2c496d..e6960a6 100644
--- a/apps/api/src/modules/order/repo.rs
+++ b/apps/api/src/modules/order/repo.rs
@@ -11,7 +11,7 @@ use super::dto::{OrderScope, OrderView};
 const ORDER_COLS: &str = "id, order_no, shop_id, user_id, status, currency, total_minor,
          discount_minor, coupon_id, shipping_address, created_at, updated_at";
 const ORDER_ITEM_COLS: &str =
-    "id, order_id, sku_id, product_name, sku_code, image, unit_price_minor, qty";
+    "id, order_id, sku_id, product_name, sku_code, image, unit_price_minor, qty, flash_sale_item_id";
 
 pub async fn attach_items(db: &PgPool, orders: Vec<Order>) -> ApiResult<Vec<OrderView>> {
     let ids: Vec<Uuid> = orders.iter().map(|o| o.id).collect();
@@ -162,6 +162,7 @@ pub async fn insert_order(
     .await?)
 }
 
+#[allow(clippy::too_many_arguments)]
 pub async fn insert_item(
     tx: &mut PgConnection,
     order_id: Uuid,
@@ -171,10 +172,11 @@ pub async fn insert_item(
     image: &Option<String>,
     unit: i64,
     qty: i32,
+    flash_sale_item_id: Option<Uuid>,
 ) -> ApiResult<()> {
     sqlx::query(
-        "INSERT INTO order_items (order_id, sku_id, product_name, sku_code, image, unit_price_minor, qty)
-         VALUES ($1, $2, $3, $4, $5, $6, $7)",
+        "INSERT INTO order_items (order_id, sku_id, product_name, sku_code, image, unit_price_minor, qty, flash_sale_item_id)
+         VALUES ($1, $2, $3, $4, $5, $6, $7, $8)",
     )
     .bind(order_id)
     .bind(sku_id)
@@ -183,6 +185,7 @@ pub async fn insert_item(
     .bind(image)
     .bind(unit)
     .bind(qty)
+    .bind(flash_sale_item_id)
     .execute(&mut *tx)
     .await?;
     Ok(())
diff --git a/apps/api/src/modules/order/service.rs b/apps/api/src/modules/order/service.rs
index 6b759fe..b0bc39d 100644
--- a/apps/api/src/modules/order/service.rs
+++ b/apps/api/src/modules/order/service.rs
@@ -6,12 +6,34 @@ use crate::error::{ApiError, ApiResult};
 use crate::http::{clamp_page, clamp_per_page, Paged};
 use crate::models::OrderStatus;
 use crate::money::convert_minor;
-use crate::modules::{cart, coupon};
+use crate::modules::{cart, coupon, flash_sale};
 use crate::state::AppState;
 
 use super::dto::{AddressBody, OrderScope, OrderView};
 use super::repo;
 
+/// One cart line after activity resolution: the standard-priced part and, when
+/// the customer is eligible, the flash-sale part. A line can therefore produce
+/// two order items, which keeps each unit-price snapshot honest.
+struct CheckoutLine {
+    shop_id: Uuid,
+    sku_id: Uuid,
+    product_name: serde_json::Value,
+    sku_code: String,
+    image: Option<String>,
+    normal_unit: i64,
+    normal_qty: i32,
+    activity_item: Option<Uuid>,
+    activity_unit: i64,
+    activity_qty: i32,
+}
+
+impl CheckoutLine {
+    fn line_total(&self) -> i64 {
+        self.normal_unit * self.normal_qty as i64 + self.activity_unit * self.activity_qty as i64
+    }
+}
+
 pub async fn list(
     state: &AppState,
     scope: OrderScope,
@@ -101,18 +123,76 @@ pub async fn checkout(
         }
     }
 
-    // Per-shop merchandise subtotal in the buyer's currency.
-    let mut subtotal_by_shop: HashMap<Uuid, i64> = HashMap::new();
-    let mut line_prices: Vec<(Uuid, i64, i32)> = Vec::new();
+    // Normal unit prices in the buyer's currency, plus the quantities the
+    // activity resolver needs.
+    let mut normal_by_sku: HashMap<Uuid, i64> = HashMap::new();
+    let mut price_qty: Vec<(Uuid, i32)> = Vec::new();
     for row in &rows {
         let qty = qty_by_sku.get(&row.sku_id).copied().unwrap_or(0);
         let from = all_currencies
             .iter()
             .find(|c| c.code == row.currency)
             .ok_or_else(|| ApiError::BadRequest(format!("currency {} disabled", row.currency)))?;
-        let unit = convert_minor(row.price_minor, from, &target)?;
-        *subtotal_by_shop.entry(row.shop_id).or_insert(0) += unit * qty as i64;
-        line_prices.push((row.sku_id, unit, qty));
+        normal_by_sku.insert(row.sku_id, convert_minor(row.price_minor, from, &target)?);
+        price_qty.push((row.sku_id, qty));
+    }
+
+    // Flash-sale eligibility is resolved and its reserved stock locked before
+    // the coupon locks, so every checkout takes locks in the same order.
+    let activities = flash_sale::service::resolve_activities(
+        &mut tx,
+        user_id,
+        &price_qty,
+        &target,
+        &all_currencies,
+    )
+    .await?;
+
+    let mut lines: Vec<CheckoutLine> = Vec::with_capacity(rows.len());
+    for row in &rows {
+        let qty = qty_by_sku.get(&row.sku_id).copied().unwrap_or(0);
+        let normal_unit = normal_by_sku.get(&row.sku_id).copied().unwrap_or(0);
+        let (activity_item, activity_unit, activity_qty) = match activities.get(&row.sku_id) {
+            Some(activity) => {
+                let eligible = activity.qty.min(qty);
+                if eligible > 0 {
+                    (Some(activity.item_id), activity.unit_price_minor, eligible)
+                } else {
+                    (None, normal_unit, 0)
+                }
+            }
+            None => (None, normal_unit, 0),
+        };
+        lines.push(CheckoutLine {
+            shop_id: row.shop_id,
+            sku_id: row.sku_id,
+            product_name: row.product_name.clone(),
+            sku_code: row.sku_code.clone(),
+            image: row.image.clone(),
+            normal_unit,
+            normal_qty: qty - activity_qty,
+            activity_item,
+            activity_unit,
+            activity_qty,
+        });
+    }
+
+    // Activity pricing and coupons are exclusive per shop order.
+    for shop_id in coupon_by_shop.keys() {
+        if lines
+            .iter()
+            .any(|line| line.shop_id == *shop_id && line.activity_qty > 0)
+        {
+            return Err(ApiError::Conflict(
+                "a coupon cannot be combined with flash-sale pricing on one shop order".into(),
+            ));
+        }
+    }
+
+    // Per-shop merchandise subtotal in the buyer's currency.
+    let mut subtotal_by_shop: HashMap<Uuid, i64> = HashMap::new();
+    for line in &lines {
+        *subtotal_by_shop.entry(line.shop_id).or_insert(0) += line.line_total();
     }
 
     // Lock every selected coupon in id order, then prove the customer owns it.
@@ -129,8 +209,8 @@ pub async fn checkout(
     let address = serde_json::to_value(&shipping_address).map_err(ApiError::internal)?;
     let mut created = Vec::new();
     for shop_id in shop_order {
-        let shop_rows: Vec<&repo::CheckoutRow> =
-            rows.iter().filter(|r| r.shop_id == shop_id).collect();
+        let shop_lines: Vec<&CheckoutLine> =
+            lines.iter().filter(|line| line.shop_id == shop_id).collect();
         let subtotal = subtotal_by_shop.get(&shop_id).copied().unwrap_or(0);
         let selected_coupon = coupon_by_shop.get(&shop_id).copied();
         // Eligibility and the discount are resolved server-side; the client
@@ -165,23 +245,40 @@ pub async fn checkout(
         if let Some(id) = selected_coupon {
             coupon::service::redeem(&mut tx, id, order.id).await?;
         }
-        for row in &shop_rows {
-            let (_, unit, qty) = line_prices
-                .iter()
-                .find(|(sku, _, _)| *sku == row.sku_id)
-                .unwrap();
-            repo::insert_item(
-                &mut tx,
-                order.id,
-                row.sku_id,
-                &row.product_name,
-                &row.sku_code,
-                &row.image,
-                *unit,
-                *qty,
-            )
-            .await?;
-            repo::decrement_stock(&mut tx, row.sku_id, *qty).await?;
+        for line in shop_lines {
+            // Activity-priced units first, then the standard-priced remainder.
+            if let (Some(item_id), true) = (line.activity_item, line.activity_qty > 0) {
+                repo::insert_item(
+                    &mut tx,
+                    order.id,
+                    line.sku_id,
+                    &line.product_name,
+                    &line.sku_code,
+                    &line.image,
+                    line.activity_unit,
+                    line.activity_qty,
+                    Some(item_id),
+                )
+                .await?;
+                flash_sale::service::consume(&mut tx, item_id, line.activity_qty).await?;
+            }
+            if line.normal_qty > 0 {
+                repo::insert_item(
+                    &mut tx,
+                    order.id,
+                    line.sku_id,
+                    &line.product_name,
+                    &line.sku_code,
+                    &line.image,
+                    line.normal_unit,
+                    line.normal_qty,
+                    None,
+                )
+                .await?;
+            }
+            // The whole ordered quantity leaves SKU stock; only the activity
+            // portion also leaves reserved activity stock.
+            repo::decrement_stock(&mut tx, line.sku_id, line.normal_qty + line.activity_qty).await?;
         }
         created.push(order);
     }
@@ -201,7 +298,9 @@ pub async fn cancel(state: &AppState, user_id: Uuid, id: Uuid) -> ApiResult<Orde
     let mut tx = state.db.begin().await?;
     let order = repo::cancel(&mut tx, user_id, id).await?;
     repo::restore_stock(&mut tx, order.id).await?;
-    // Stock and the redeemed coupon come back in the same transaction.
+    // Reserved activity stock, SKU stock, and the redeemed coupon all come back
+    // in the same transaction.
+    flash_sale::service::restore_for_order(&mut tx, order.id).await?;
     coupon::service::restore_for_order(&mut tx, order.id).await?;
     tx.commit().await?;
     let mut views = repo::attach_items(&state.db, vec![order]).await?;
diff --git a/apps/api/tests/flash_sales.rs b/apps/api/tests/flash_sales.rs
new file mode 100644
index 0000000..9e9f993
--- /dev/null
+++ b/apps/api/tests/flash_sales.rs
@@ -0,0 +1,399 @@
+mod common;
+
+use std::collections::HashMap;
+
+use common::{
+    add_to_cart, client, login_admin, register_customer, setup_sellable, spawn_app, TestApp,
+};
+use serial_test::serial;
+use uuid::Uuid;
+
+fn past_window() -> (String, String) {
+    ("2020-01-01T00:00:00Z".into(), "2020-01-02T00:00:00Z".into())
+}
+
+fn active_window() -> (String, String) {
+    ("2020-01-01T00:00:00Z".into(), "2999-01-01T00:00:00Z".into())
+}
+
+async fn create_session(app: &TestApp, owner: &str, starts: &str, ends: &str) -> String {
+    let res = client()
+        .post(app.url("/api/shop/flash-sales"))
+        .bearer_auth(owner)
+        .json(&serde_json::json!({
+            "label": {"en": "Flash", "zh": "秒杀"},
+            "starts_at": starts,
+            "ends_at": ends,
+            "enabled": true,
+        }))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(res.status(), 201, "create session: {:?}", res.text().await);
+    res.json::<serde_json::Value>().await.unwrap()["id"]
+        .as_str()
+        .unwrap()
+        .to_string()
+}
+
+async fn add_item(
+    app: &TestApp,
+    owner: &str,
+    session_id: &str,
+    sku_id: &str,
+    sale_price: i64,
+    reserved: i32,
+    limit: i32,
+) -> reqwest::Response {
+    client()
+        .post(app.url(&format!("/api/shop/flash-sales/{session_id}/items")))
+        .bearer_auth(owner)
+        .json(&serde_json::json!({
+            "sku_id": sku_id,
+            "sale_price_minor": sale_price,
+            "currency": "USD",
+            "reserved_stock": reserved,
+            "per_customer_limit": limit,
+        }))
+        .send()
+        .await
+        .unwrap()
+}
+
+async fn checkout_with(
+    app: &TestApp,
+    token: &str,
+    coupon_by_shop: HashMap<String, String>,
+) -> reqwest::Response {
+    client()
+        .post(app.url("/api/orders/checkout"))
+        .bearer_auth(token)
+        .json(&serde_json::json!({
+            "shipping_address": {
+                "recipient": "Test", "phone": "1", "country": "US",
+                "region": "CA", "city": "SF", "line1": "1 Way", "postal_code": "94105"
+            },
+            "currency": "USD",
+            "coupon_by_shop": coupon_by_shop,
+        }))
+        .send()
+        .await
+        .unwrap()
+}
+
+async fn create_template(app: &TestApp, owner: &str, amount: i64, threshold: i64) -> String {
+    let res = client()
+        .post(app.url("/api/shop/coupon-templates"))
+        .bearer_auth(owner)
+        .json(&serde_json::json!({
+            "title": {"en": "Coupon", "zh": "优惠券"},
+            "amount_minor": amount,
+            "threshold_minor": threshold,
+            "currency": "USD",
+            "stock": 5,
+            "enabled": true,
+            "starts_at": "2020-01-01T00:00:00Z",
+            "ends_at": "2999-01-01T00:00:00Z",
+        }))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(res.status(), 201, "create template: {:?}", res.text().await);
+    res.json::<serde_json::Value>().await.unwrap()["id"]
+        .as_str()
+        .unwrap()
+        .to_string()
+}
+
+async fn claim(app: &TestApp, token: &str, template_id: &str) -> String {
+    let res = client()
+        .post(app.url("/api/me/coupons"))
+        .bearer_auth(token)
+        .json(&serde_json::json!({ "template_id": template_id }))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(res.status(), 201, "claim: {:?}", res.text().await);
+    res.json::<serde_json::Value>().await.unwrap()["id"]
+        .as_str()
+        .unwrap()
+        .to_string()
+}
+
+async fn flash_item_state(app: &TestApp, item_id: &str) -> (i32, i32) {
+    sqlx::query_as("SELECT reserved_stock, sold_count FROM flash_sale_items WHERE id = $1")
+        .bind(Uuid::parse_str(item_id).unwrap())
+        .fetch_one(&app.db)
+        .await
+        .unwrap()
+}
+
+async fn activity_lines(app: &TestApp, order_id: &str) -> i64 {
+    sqlx::query_scalar(
+        "SELECT count(*) FROM order_items
+          WHERE order_id = $1 AND flash_sale_item_id IS NOT NULL",
+    )
+    .bind(Uuid::parse_str(order_id).unwrap())
+    .fetch_one(&app.db)
+    .await
+    .unwrap()
+}
+
+#[tokio::test]
+#[serial]
+async fn inactive_window_falls_back_to_normal_price() {
+    let app = spawn_app().await;
+    let admin = login_admin(&app).await;
+    let (owner, _shop, _product, sku) = setup_sellable(&app, &admin, "fs-past", 1000, 5).await;
+    let (starts, ends) = past_window();
+    let session = create_session(&app, &owner, &starts, &ends).await;
+    let res = add_item(&app, &owner, &session, &sku, 500, 5, 3).await;
+    assert_eq!(res.status(), 201, "past sessions may still be configured");
+
+    // Not discoverable as active.
+    let active: Vec<serde_json::Value> = client()
+        .get(app.url("/api/flash-sales"))
+        .send()
+        .await
+        .unwrap()
+        .json()
+        .await
+        .unwrap();
+    assert!(active.is_empty(), "an elapsed session is not public");
+
+    let (token, _) = register_customer(&app, "fs-past").await;
+    add_to_cart(&app, &token, &sku, 1).await;
+    let res = checkout_with(&app, &token, HashMap::new()).await;
+    assert_eq!(res.status(), 201);
+    let orders: Vec<serde_json::Value> = res.json().await.unwrap();
+    let item = &orders[0]["items"][0];
+    assert_eq!(item["unit_price_minor"], 1000, "normal price applies");
+    assert!(item["flash_sale_item_id"].is_null());
+    assert_eq!(activity_lines(&app, orders[0]["id"].as_str().unwrap()).await, 0);
+}
+
+#[tokio::test]
+#[serial]
+async fn cross_shop_sku_and_overlapping_sessions_are_rejected() {
+    let app = spawn_app().await;
+    let admin = login_admin(&app).await;
+    let (owner_a, shop_a, _product_a, sku_a) = setup_sellable(&app, &admin, "fs-a", 1000, 5).await;
+    let (_owner_b, _shop_b, _product_b, sku_b) = setup_sellable(&app, &admin, "fs-b", 1000, 5).await;
+    let (starts, ends) = active_window();
+
+    let session = create_session(&app, &owner_a, &starts, &ends).await;
+    let res = add_item(&app, &owner_a, &session, &sku_b, 500, 5, 3).await;
+    assert_eq!(res.status(), 400, "another shop's SKU is refused");
+
+    let res = add_item(&app, &owner_a, &session, &sku_a, 500, 5, 3).await;
+    assert_eq!(res.status(), 201);
+
+    // A second overlapping session cannot list the same SKU.
+    let session_two = create_session(&app, &owner_a, &starts, &ends).await;
+    let res = add_item(&app, &owner_a, &session_two, &sku_a, 400, 5, 3).await;
+    assert_eq!(res.status(), 409, "overlapping sale for the same SKU is refused");
+    let _ = shop_a;
+}
+
+#[tokio::test]
+#[serial]
+async fn overlapping_group_buying_is_rejected_once_that_capability_exists() {
+    let app = spawn_app().await;
+    let table_exists: bool = sqlx::query_scalar(
+        "SELECT to_regclass('public.group_buying_activities') IS NOT NULL",
+    )
+    .fetch_one(&app.db)
+    .await
+    .unwrap();
+    if !table_exists {
+        // add-group-buying lands after add-flash-sales, so this cross-capability
+        // scenario can only be exercised once its table exists.
+        eprintln!("skipped: group_buying_activities is not present yet");
+        return;
+    }
+
+    let admin = login_admin(&app).await;
+    let (owner, _shop, _product, sku) = setup_sellable(&app, &admin, "fs-gb", 1000, 5).await;
+    let (starts, ends) = active_window();
+    sqlx::query(
+        "INSERT INTO group_buying_activities
+             (shop_id, sku_id, name, group_price_minor, currency, required_members,
+              starts_at, ends_at, group_lifetime_hours)
+         SELECT p.shop_id, $1, '{\"en\":\"g\",\"zh\":\"团\"}', 100, 'USD', 2, $2, $3, 24
+           FROM skus s JOIN products p ON p.id = s.product_id WHERE s.id = $1",
+    )
+    .bind(Uuid::parse_str(&sku).unwrap())
+    .bind(chrono::DateTime::parse_from_rfc3339(&starts).unwrap())
+    .bind(chrono::DateTime::parse_from_rfc3339(&ends).unwrap())
+    .execute(&app.db)
+    .await
+    .unwrap();
+
+    let session = create_session(&app, &owner, &starts, &ends).await;
+    let res = add_item(&app, &owner, &session, &sku, 500, 5, 3).await;
+    assert_eq!(res.status(), 409, "overlapping group-buy SKU is refused");
+}
+
+#[tokio::test]
+#[serial]
+async fn reserved_stock_admits_one_activity_price() {
+    let app = spawn_app().await;
+    let admin = login_admin(&app).await;
+    let (owner, _shop, _product, sku) = setup_sellable(&app, &admin, "fs-race", 1000, 10).await;
+    let (starts, ends) = active_window();
+    let session = create_session(&app, &owner, &starts, &ends).await;
+    let res = add_item(&app, &owner, &session, &sku, 400, 1, 5).await;
+    assert_eq!(res.status(), 201);
+    let item_id = res.json::<serde_json::Value>().await.unwrap()["id"]
+        .as_str()
+        .unwrap()
+        .to_string();
+
+    let (token_a, _) = register_customer(&app, "fs-race-a").await;
+    let (token_b, _) = register_customer(&app, "fs-race-b").await;
+    add_to_cart(&app, &token_a, &sku, 1).await;
+    add_to_cart(&app, &token_b, &sku, 1).await;
+
+    let (a, b) = tokio::join!(
+        checkout_with(&app, &token_a, HashMap::new()),
+        checkout_with(&app, &token_b, HashMap::new()),
+    );
+    assert!(a.status().is_success() && b.status().is_success());
+
+    let (reserved, sold) = flash_item_state(&app, &item_id).await;
+    assert_eq!(reserved, 0, "the single reserved unit is consumed");
+    assert_eq!(sold, 1);
+
+    let activity: i64 = sqlx::query_scalar(
+        "SELECT count(*) FROM order_items WHERE flash_sale_item_id = $1",
+    )
+    .bind(Uuid::parse_str(&item_id).unwrap())
+    .fetch_one(&app.db)
+    .await
+    .unwrap();
+    assert_eq!(activity, 1, "exactly one line got the activity price");
+}
+
+#[tokio::test]
+#[serial]
+async fn per_customer_limit_splits_the_line() {
+    let app = spawn_app().await;
+    let admin = login_admin(&app).await;
+    let (owner, _shop, _product, sku) = setup_sellable(&app, &admin, "fs-limit", 1000, 10).await;
+    let (starts, ends) = active_window();
+    let session = create_session(&app, &owner, &starts, &ends).await;
+    let res = add_item(&app, &owner, &session, &sku, 400, 10, 1).await;
+    assert_eq!(res.status(), 201);
+    let item_id = res.json::<serde_json::Value>().await.unwrap()["id"]
+        .as_str()
+        .unwrap()
+        .to_string();
+
+    let (token, _) = register_customer(&app, "fs-limit").await;
+    add_to_cart(&app, &token, &sku, 3).await;
+    let res = checkout_with(&app, &token, HashMap::new()).await;
+    assert_eq!(res.status(), 201);
+    let orders: Vec<serde_json::Value> = res.json().await.unwrap();
+    let items = orders[0]["items"].as_array().unwrap();
+    assert_eq!(items.len(), 2, "one activity line and one standard line");
+    let activity: Vec<&serde_json::Value> = items
+        .iter()
+        .filter(|i| !i["flash_sale_item_id"].is_null())
+        .collect();
+    let standard: Vec<&serde_json::Value> = items
+        .iter()
+        .filter(|i| i["flash_sale_item_id"].is_null())
+        .collect();
+    assert_eq!(activity.len(), 1);
+    assert_eq!(activity[0]["qty"], 1, "only the allowance gets the activity price");
+    assert_eq!(activity[0]["unit_price_minor"], 400);
+    assert_eq!(standard.len(), 1);
+    assert_eq!(standard[0]["qty"], 2);
+    assert_eq!(standard[0]["unit_price_minor"], 1000);
+    assert_eq!(orders[0]["total_minor"], 400 + 2000);
+
+    // The allowance is spent, so a second checkout is standard-priced.
+    add_to_cart(&app, &token, &sku, 1).await;
+    let res = checkout_with(&app, &token, HashMap::new()).await;
+    let orders: Vec<serde_json::Value> = res.json().await.unwrap();
+    assert!(orders[0]["items"][0]["flash_sale_item_id"].is_null());
+    assert_eq!(activity_lines(&app, orders[0]["id"].as_str().unwrap()).await, 0);
+
+    let (_, sold) = flash_item_state(&app, &item_id).await;
+    assert_eq!(sold, 1);
+}
+
+#[tokio::test]
+#[serial]
+async fn coupon_is_rejected_on_a_flash_priced_shop_order() {
+    let app = spawn_app().await;
+    let admin = login_admin(&app).await;
+    let (owner, shop, _product, sku) = setup_sellable(&app, &admin, "fs-cp", 1000, 5).await;
+    let (starts, ends) = active_window();
+    let session = create_session(&app, &owner, &starts, &ends).await;
+    assert_eq!(add_item(&app, &owner, &session, &sku, 400, 5, 5).await.status(), 201);
+
+    let template = create_template(&app, &owner, 100, 0).await;
+    let (token, _) = register_customer(&app, "fs-cp").await;
+    let coupon = claim(&app, &token, &template).await;
+
+    add_to_cart(&app, &token, &sku, 1).await;
+    let res = checkout_with(&app, &token, HashMap::from([(shop, coupon)])).await;
+    assert_eq!(res.status(), 409, "activity pricing and coupons are exclusive");
+}
+
+#[tokio::test]
+#[serial]
+async fn cancel_restores_reserved_stock_and_coupon() {
+    let app = spawn_app().await;
+    let admin = login_admin(&app).await;
+    let (flash_owner, _flash_shop, _p1, flash_sku) =
+        setup_sellable(&app, &admin, "fs-cancel-a", 1000, 5).await;
+    let (owner_b, shop_b, _p2, sku_b) = setup_sellable(&app, &admin, "fs-cancel-b", 2000, 5).await;
+    let (starts, ends) = active_window();
+    let session = create_session(&app, &flash_owner, &starts, &ends).await;
+    let res = add_item(&app, &flash_owner, &session, &flash_sku, 400, 3, 5).await;
+    assert_eq!(res.status(), 201);
+    let item_id = res.json::<serde_json::Value>().await.unwrap()["id"]
+        .as_str()
+        .unwrap()
+        .to_string();
+
+    let template = create_template(&app, &owner_b, 100, 0).await;
+    let (token, _) = register_customer(&app, "fs-cancel").await;
+    let coupon = claim(&app, &token, &template).await;
+
+    add_to_cart(&app, &token, &flash_sku, 2).await;
+    add_to_cart(&app, &token, &sku_b, 1).await;
+    let res = checkout_with(&app, &token, HashMap::from([(shop_b, coupon.clone())])).await;
+    assert_eq!(res.status(), 201, "checkout: {:?}", res.text().await);
+    let orders: Vec<serde_json::Value> = res.json().await.unwrap();
+    assert_eq!(orders.len(), 2);
+
+    let (reserved_after, sold_after) = flash_item_state(&app, &item_id).await;
+    assert_eq!(reserved_after, 1, "2 of 3 reserved units consumed");
+    assert_eq!(sold_after, 2);
+
+    for order in &orders {
+        let res = client()
+            .post(app.url(&format!("/api/orders/{}/cancel", order["id"].as_str().unwrap())))
+            .bearer_auth(&token)
+            .send()
+            .await
+            .unwrap();
+        assert_eq!(res.status(), 200);
+    }
+
+    let (reserved, sold) = flash_item_state(&app, &item_id).await;
+    assert_eq!(reserved, 3, "reserved activity stock is restored");
+    assert_eq!(sold, 0);
+
+    let (status, order_ref): (String, Option<Uuid>) =
+        sqlx::query_as("SELECT status::text, order_id FROM coupons WHERE id = $1")
+            .bind(Uuid::parse_str(&coupon).unwrap())
+            .fetch_one(&app.db)
+            .await
+            .unwrap();
+    assert_eq!(status, "claimed");
+    assert!(order_ref.is_none());
+}
diff --git a/openspec/changes/add-flash-sales/tasks.md b/openspec/changes/add-flash-sales/tasks.md
index 3c78f08..f8a3928 100644
--- a/openspec/changes/add-flash-sales/tasks.md
+++ b/openspec/changes/add-flash-sales/tasks.md
@@ -1,14 +1,14 @@
 ## 1. Flash-sale domain and contract
 
-- [ ] 1.1 Add additive migrations for shop flash-sale sessions, SKU activity items, reserved stock, per-customer limits, and order-item activity snapshots.
-- [ ] 1.2 Implement the Rust flash-sale module with shop-scoped session/item management and public active-session discovery.
+- [x] 1.1 Add additive migrations for shop flash-sale sessions, SKU activity items, reserved stock, per-customer limits, and order-item activity snapshots.
+- [x] 1.2 Implement the Rust flash-sale module with shop-scoped session/item management and public active-session discovery.
 - [ ] 1.3 Add shared flash-sale types, client methods, localized strings, and fixed-data adapter parity.
 
 ## 2. Checkout price and inventory resolution
 
-- [ ] 2.1 Resolve active eligible activity items server-side during checkout and snapshot final line pricing and activity identity. Reject coupon selection on any shop order that applied flash pricing; restate coupon persistence for non-flash shop orders.
-- [ ] 2.2 Conditionally decrement and restore activity inventory with SKU inventory in deterministic lock order; enforce the per-customer limit. Pending-payment cancellation restores both inventories and any redeemed coupon.
-- [ ] 2.3 Add integration tests for inactive-window fallback, cross-shop configuration rejection, overlapping group-buy SKU rejection, final reserved-stock contention, customer limits, coupon rejection on flash-priced shop orders, coupon restore on cancel, and cancellation restoration.
+- [x] 2.1 Resolve active eligible activity items server-side during checkout and snapshot final line pricing and activity identity. Reject coupon selection on any shop order that applied flash pricing; restate coupon persistence for non-flash shop orders.
+- [x] 2.2 Conditionally decrement and restore activity inventory with SKU inventory in deterministic lock order; enforce the per-customer limit. Pending-payment cancellation restores both inventories and any redeemed coupon.
+- [x] 2.3 Add integration tests for inactive-window fallback, cross-shop configuration rejection, overlapping group-buy SKU rejection, final reserved-stock contention, customer limits, coupon rejection on flash-priced shop orders, coupon restore on cancel, and cancellation restoration.
 
 ## 3. Merchant and mall surfaces
 
@@ -19,4 +19,4 @@
 ## 4. Verification and specification
 
 - [ ] 4.1 Seed an active deterministic flash sale and browser-smoke its discovery and checkout pricing path.
-- [ ] 4.2 Run cargo test for vmall-api, builds for mall and shop-admin, and strict validation for this OpenSpec change.
\ No newline at end of file
+- [ ] 4.2 Run cargo test for vmall-api, builds for mall and shop-admin, and strict validation for this OpenSpec change.