fix(backend): 修复代码审核发现的 8 个确认问题

- P0-1: subscription_maintenance_queue 使用 RWMutex 防止 channel close/send 竞态
- P0-2: billing_service CalculateCostWithLongContext 修复被吞没的 out-range 错误
- P1-1: timing_wheel_service Schedule/ScheduleRecurring 添加 SetTimer 错误日志
- P1-2: sora_gateway_service StoreFromURLs 失败时降级使用原始 URL
- P1-3: concurrency_cache 用 Pipeline 替代 Lua 脚本兼容 Redis Cluster
- P1-6: sora_media_cleanup_service runCleanup 添加 nil cfg/storage 防护

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/billing_service.go

@@ -297,7 +297,7 @@ func (s *BillingService) CalculateCostWithLongContext(model string, tokens Usage
 	}
 	outRangeCost, err := s.CalculateCost(model, outRangeTokens, rateMultiplier*extraMultiplier)
 	if err != nil {
-		return inRangeCost, nil // 出错时返回范围内成本
+		return inRangeCost, fmt.Errorf("out-range cost: %w", err)
 	}
 
 	// 合并成本