perf(wallet): 避免同钱包预留占满数据库池

千并发验收的 pg_stat_activity 显示大量事务锁等待集中在同一验收钱包，等待行锁的请求持续占用 API Pool，导致 canceled acquire 和排队增长。

在每个 Store 内增加 256 槽钱包预留分片锁，同一钱包先在进程内串行再申请数据库连接；跨 API 节点仍由 PostgreSQL 行锁保证余额与幂等正确性。

验证：Go 全量测试、临时 PostgreSQL 并发账务集成测试、gofmt 和 git diff --check 通过。

apps/api/internal/store/wallet_reservation_test.go

@@ -113,6 +113,31 @@ func TestReserveTaskBillingSerializesConcurrentWalletReservations(t *testing.T)
 	}
 }
 
+func TestWalletReservationLockSetSerializesSameWallet(t *testing.T) {
+	var locks walletReservationLockSet
+	unlock := locks.lock("wallet-user")
+	acquired := make(chan struct{})
+	released := make(chan struct{})
+	go func() {
+		defer close(released)
+		unlockSecond := locks.lock("wallet-user")
+		close(acquired)
+		unlockSecond()
+	}()
+	select {
+	case <-acquired:
+		t.Fatal("same-wallet reservation lock was acquired concurrently")
+	case <-time.After(20 * time.Millisecond):
+	}
+	unlock()
+	select {
+	case <-acquired:
+	case <-time.After(time.Second):
+		t.Fatal("same-wallet reservation lock did not unblock")
+	}
+	<-released
+}
+
 func seedWalletReservationUser(t *testing.T, ctx context.Context, db *Store) (string, string) {
 	t.Helper()
 	suffix := strconv.FormatInt(time.Now().UnixNano(), 10)