feat(api): add customer accounts with live summary and append-only ledger

One balance row per (user, kind, currency): available and frozen carry the
platform base currency, points carries none. Monetary and points rows use
separate partial unique indexes because a plain UNIQUE lets NULL repeat.

Balance changes go through transactional primitives that debits guard with a
conditional update, credits add atomically, and freeze/release move both sides
in one transaction after locking rows by primary key. Every change appends an
immutable entry holding its resulting balance. Registration and a migration
backfill create the zero rows; GET /api/me/stats is the only public surface and
no endpoint mutates a balance.

The mall buyer center and points page drop the USER_STATS fixture for the
shared contract; the fixture stays exported so the fixed-data adapter can still
serve the account domain as a rollback path.

Implements openspec change add-customer-accounts.

apps/api/src/models.rs

@@ -242,6 +242,50 @@ pub struct Invoice {
     pub created_at: DateTime<Utc>,
 }
 
+/// Balance bucket. `available`/`frozen` are monetary and carry a currency;
+/// `points` is an integer loyalty balance with no currency.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Type)]
+#[sqlx(type_name = "customer_account_kind", rename_all = "snake_case")]
+#[serde(rename_all = "snake_case")]
+pub enum AccountKind {
+    Available,
+    Frozen,
+    Points,
+}
+
+impl AccountKind {
+    pub fn is_monetary(self) -> bool {
+        !matches!(self, AccountKind::Points)
+    }
+}
+
+#[derive(Debug, Clone, sqlx::FromRow)]
+pub struct CustomerAccount {
+    pub id: Uuid,
+    pub user_id: Uuid,
+    pub kind: AccountKind,
+    pub currency: Option<String>,
+    pub balance_minor: i64,
+}
+
+pub const CUSTOMER_ACCOUNT_COLUMNS: &str = "id, user_id, kind, currency, balance_minor";
+
+/// One immutable balance movement. Written in the same transaction as the
+/// guarded account update it describes.
+#[derive(Debug, Clone, Serialize, sqlx::FromRow)]
+pub struct CustomerAccountEntry {
+    pub id: Uuid,
+    pub account_id: Uuid,
+    pub delta_minor: i64,
+    pub balance_minor: i64,
+    pub reason: String,
+    pub reference_type: Option<String>,
+    pub reference_id: Option<Uuid>,
+    pub created_at: DateTime<Utc>,
+}
+
+pub const CUSTOMER_ACCOUNT_ENTRY_COLUMNS: &str = "id, account_id, delta_minor, balance_minor, reason, reference_type, reference_id, created_at";
+
 #[derive(Debug, Clone, Serialize, sqlx::FromRow)]
 pub struct AddressBookEntry {
     pub id: Uuid,