feat(api): flash sales resolved server-side at checkout

Shop-owned timed sessions carry SKU activity items with their own reserved
inventory and per-customer limit. Checkout resolves eligibility on the server,
locks candidate items by primary key after the SKU locks, and splits a cart line
into an activity-priced item plus a standard-priced remainder, so every unit
price snapshot is honest and a customer cannot exceed the limit.

Reserved activity stock and SKU stock decrement together under guards, and a
pending-payment cancellation restores both plus any redeemed coupon. Coupons are
rejected on a shop order that applied activity pricing, and a SKU cannot join
two overlapping enabled sessions.

The overlap check against group buying is present but dormant: that capability
lands later, so the check activates only once its table exists.

Surfaces (shop-admin, mall) and seeding follow.

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);