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/timing_wheel_service.go

@@ -47,7 +47,9 @@ func (s *TimingWheelService) Stop() {
 
 // Schedule schedules a one-time task
 func (s *TimingWheelService) Schedule(name string, delay time.Duration, fn func()) {
-	_ = s.tw.SetTimer(name, fn, delay)
+	if err := s.tw.SetTimer(name, fn, delay); err != nil {
+		log.Printf("[TimingWheel] SetTimer failed for %q: %v", name, err)
+	}
 }
 
 // ScheduleRecurring schedules a recurring task
@@ -55,9 +57,13 @@ func (s *TimingWheelService) ScheduleRecurring(name string, interval time.Durati
 	var schedule func()
 	schedule = func() {
 		fn()
-		_ = s.tw.SetTimer(name, schedule, interval)
+		if err := s.tw.SetTimer(name, schedule, interval); err != nil {
+			log.Printf("[TimingWheel] recurring SetTimer failed for %q: %v", name, err)
+		}
+	}
+	if err := s.tw.SetTimer(name, schedule, interval); err != nil {
+		log.Printf("[TimingWheel] initial SetTimer failed for %q: %v", name, err)
 	}
-	_ = s.tw.SetTimer(name, schedule, interval)
 }
 
 // Cancel cancels a scheduled task