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/modules/mod.rs

@@ -1,3 +1,4 @@
+pub mod account;
 pub mod address;
 pub mod billing;
 pub mod cart;
@@ -17,6 +18,7 @@ use crate::state::AppState;
 pub fn api_router() -> Router<AppState> {
     Router::new()
         .merge(health::router())
+        .merge(account::router())
         .merge(address::router())
         .merge(identity::router())
         .merge(currency::router())