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.go

@@ -5,14 +5,30 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"hash/fnv"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/easyai/easyai-ai-gateway/apps/api/internal/auth"
 	"github.com/jackc/pgx/v5"
 )
 
+const walletReservationLockStripes = 256
+
+type walletReservationLockSet struct {
+	stripes [walletReservationLockStripes]sync.Mutex
+}
+
+func (locks *walletReservationLockSet) lock(gatewayUserID string) func() {
+	hasher := fnv.New32a()
+	_, _ = hasher.Write([]byte(strings.TrimSpace(gatewayUserID)))
+	stripe := &locks.stripes[hasher.Sum32()%walletReservationLockStripes]
+	stripe.Lock()
+	return stripe.Unlock
+}
+
 type GatewayWalletAccount struct {
 	ID              string         `json:"id"`
 	GatewayTenantID string         `json:"gatewayTenantId,omitempty"`
@@ -153,12 +169,16 @@ func (s *Store) ReserveTaskBilling(ctx context.Context, task GatewayTask, user *
 		pricingSnapshot = emptyObjectIfNil(pricingSnapshots[0])
 	}
 	if exactAmount := walletString(pricingSnapshot["reservationAmount"]); exactAmount != "" {
+		unlock := s.walletReservationLocks.lock(gatewayUserID)
+		defer unlock()
 		return s.reserveTaskBillingExact(ctx, task, gatewayUserID, exactAmount, pricingSnapshot)
 	}
 	amounts := walletBillingAmounts(billings)
 	if len(amounts) == 0 {
 		return nil, nil
 	}
+	unlock := s.walletReservationLocks.lock(gatewayUserID)
+	defer unlock()
 
 	reservations := make([]WalletBillingReservation, 0, len(amounts))
 	pricingSnapshotJSON, _ := json.Marshal(sanitizeJSONForStorage(pricingSnapshot))