feat: Schedule batch update for account last_used_at

Implement deferred batch update mechanism to reduce database load:

- Add DeferredService for batching account last_used_at updates
- Add TimingWheelService for efficient recurring task scheduling
- Integrate with GatewayService and OpenAIGatewayService
- Implement BatchUpdateLastUsed repository method using CASE...WHEN SQL
- Fix golangci-lint error: Replace interface{} with any

Benefits:
- Reduces database writes by batching updates (10-second intervals)
- Improves request throughput by deferring non-critical updates
- Maintains accurate account usage tracking for scheduling

backend/internal/service/account_service.go

@@ -29,6 +29,7 @@ type AccountRepository interface {
 	ListByPlatform(ctx context.Context, platform string) ([]Account, error)
 
 	UpdateLastUsed(ctx context.Context, id int64) error
+	BatchUpdateLastUsed(ctx context.Context, updates map[int64]time.Time) error
 	SetError(ctx context.Context, id int64, errorMsg string) error
 	SetSchedulable(ctx context.Context, id int64, schedulable bool) error
 	BindGroups(ctx context.Context, accountID int64, groupIDs []int64) error