feat(gateway): 实现负载感知的账号调度优化

- 新增调度配置：粘性会话排队、兜底排队、负载计算、槽位清理
- 实现账号级等待队列和批量负载查询（Redis Lua 脚本）
- 三层选择策略：粘性会话优先 → 负载感知选择 → 兜底排队
- 后台定期清理过期槽位，防止资源泄漏
- 集成到所有网关处理器（Claude/Gemini/OpenAI）

backend/internal/service/wire.go

@@ -73,6 +73,15 @@ func ProvideDeferredService(accountRepo AccountRepository, timingWheel *TimingWh
 	return svc
 }
 
+// ProvideConcurrencyService creates ConcurrencyService and starts slot cleanup worker.
+func ProvideConcurrencyService(cache ConcurrencyCache, accountRepo AccountRepository, cfg *config.Config) *ConcurrencyService {
+	svc := NewConcurrencyService(cache)
+	if cfg != nil {
+		svc.StartSlotCleanupWorker(accountRepo, cfg.Gateway.Scheduling.SlotCleanupInterval)
+	}
+	return svc
+}
+
 // ProviderSet is the Wire provider set for all services
 var ProviderSet = wire.NewSet(
 	// Core services
@@ -107,7 +116,7 @@ var ProviderSet = wire.NewSet(
 	ProvideEmailQueueService,
 	NewTurnstileService,
 	NewSubscriptionService,
-	NewConcurrencyService,
+	ProvideConcurrencyService,
 	NewIdentityService,
 	NewCRSSyncService,
 	ProvideUpdateService,