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/deferred_service.go

@@ -0,0 +1,76 @@
+package service
+
+import (
+	"context"
+	"log"
+	"sync"
+	"time"
+)
+
+// DeferredService provides deferred batch update functionality
+type DeferredService struct {
+	accountRepo AccountRepository
+	timingWheel *TimingWheelService
+	interval    time.Duration
+
+	lastUsedUpdates sync.Map
+}
+
+// NewDeferredService creates a new DeferredService instance
+func NewDeferredService(accountRepo AccountRepository, timingWheel *TimingWheelService, interval time.Duration) *DeferredService {
+	return &DeferredService{
+		accountRepo: accountRepo,
+		timingWheel: timingWheel,
+		interval:    interval,
+	}
+}
+
+// Start starts the deferred service
+func (s *DeferredService) Start() {
+	s.timingWheel.ScheduleRecurring("deferred:last_used", s.interval, s.flushLastUsed)
+	log.Printf("[DeferredService] Started (interval: %v)", s.interval)
+}
+
+// Stop stops the deferred service
+func (s *DeferredService) Stop() {
+	s.timingWheel.Cancel("deferred:last_used")
+	s.flushLastUsed()
+	log.Printf("[DeferredService] Service stopped")
+}
+
+func (s *DeferredService) ScheduleLastUsedUpdate(accountID int64) {
+	s.lastUsedUpdates.Store(accountID, time.Now())
+}
+
+func (s *DeferredService) flushLastUsed() {
+	updates := make(map[int64]time.Time)
+	s.lastUsedUpdates.Range(func(key, value any) bool {
+		id, ok := key.(int64)
+		if !ok {
+			return true
+		}
+		ts, ok := value.(time.Time)
+		if !ok {
+			return true
+		}
+		updates[id] = ts
+		s.lastUsedUpdates.Delete(key)
+		return true
+	})
+
+	if len(updates) == 0 {
+		return
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	if err := s.accountRepo.BatchUpdateLastUsed(ctx, updates); err != nil {
+		log.Printf("[DeferredService] BatchUpdateLastUsed failed (%d accounts): %v", len(updates), err)
+		for id, ts := range updates {
+			s.lastUsedUpdates.Store(id, ts)
+		}
+	} else {
+		log.Printf("[DeferredService] BatchUpdateLastUsed flushed %d accounts", len(updates))
+	}
+}