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/sora_media_cleanup_service.go

@@ -85,6 +85,9 @@ func (s *SoraMediaCleanupService) Stop() {
 }
 
 func (s *SoraMediaCleanupService) runCleanup() {
+	if s.cfg == nil || s.storage == nil {
+		return
+	}
 	retention := s.cfg.Sora.Storage.Cleanup.RetentionDays
 	if retention <= 0 {
 		log.Printf("[SoraCleanup] skipped (retention_days=%d)", retention)