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/mall/mock/api.ts

@@ -4,6 +4,7 @@
 
 import { ApiError } from "@vmall/shared";
 import type {
+  AccountSummary,
   Address,
   AddressBookEntry,
   AddressInput,
@@ -30,6 +31,7 @@ import {
   MOCK_QUICK_LINKS,
   MOCK_STORES,
   MOCK_USER,
+  USER_STATS,
   MOCK_ADDRESSES,
   defaultAddress,
   mockConvertMinor,
@@ -179,6 +181,13 @@ export function createMockApi(): ApiClient {
     register: () => Promise.resolve(tokens()),
     login: () => Promise.resolve(tokens()),
     me: (): Promise<User> => Promise.resolve(MOCK_USER),
+    getAccountSummary: (): Promise<AccountSummary> =>
+      Promise.resolve({
+        balance_minor: USER_STATS.balanceMinor,
+        frozen_minor: USER_STATS.frozenMinor,
+        currency: BASE_CURRENCY,
+        points: USER_STATS.points,
+      }),
 
     listProducts: (q = {}) =>
       Promise.resolve(