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/timing_wheel_service.go

@@ -0,0 +1,63 @@
+package service
+
+import (
+	"log"
+	"sync"
+	"time"
+
+	"github.com/zeromicro/go-zero/core/collection"
+)
+
+// TimingWheelService wraps go-zero's TimingWheel for task scheduling
+type TimingWheelService struct {
+	tw       *collection.TimingWheel
+	stopOnce sync.Once
+}
+
+// NewTimingWheelService creates a new TimingWheelService instance
+func NewTimingWheelService() *TimingWheelService {
+	// 1 second tick, 3600 slots = supports up to 1 hour delay
+	// execute function: runs func() type tasks
+	tw, err := collection.NewTimingWheel(1*time.Second, 3600, func(key, value any) {
+		if fn, ok := value.(func()); ok {
+			fn()
+		}
+	})
+	if err != nil {
+		panic(err)
+	}
+	return &TimingWheelService{tw: tw}
+}
+
+// Start starts the timing wheel
+func (s *TimingWheelService) Start() {
+	log.Println("[TimingWheel] Started (auto-start by go-zero)")
+}
+
+// Stop stops the timing wheel
+func (s *TimingWheelService) Stop() {
+	s.stopOnce.Do(func() {
+		s.tw.Stop()
+		log.Println("[TimingWheel] Stopped")
+	})
+}
+
+// Schedule schedules a one-time task
+func (s *TimingWheelService) Schedule(name string, delay time.Duration, fn func()) {
+	_ = s.tw.SetTimer(name, fn, delay)
+}
+
+// ScheduleRecurring schedules a recurring task
+func (s *TimingWheelService) ScheduleRecurring(name string, interval time.Duration, fn func()) {
+	var schedule func()
+	schedule = func() {
+		fn()
+		_ = s.tw.SetTimer(name, schedule, interval)
+	}
+	_ = s.tw.SetTimer(name, schedule, interval)
+}
+
+// Cancel cancels a scheduled task
+func (s *TimingWheelService) Cancel(name string) {
+	_ = s.tw.RemoveTimer(name)
+}