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_gateway_service.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log"
 	"mime"
 	"net"
 	"net/http"
@@ -210,9 +211,11 @@ func (s *SoraGatewayService) Forward(ctx context.Context, c *gin.Context, accoun
 	if len(mediaURLs) > 0 && s.mediaStorage != nil && s.mediaStorage.Enabled() {
 		stored, storeErr := s.mediaStorage.StoreFromURLs(reqCtx, mediaType, mediaURLs)
 		if storeErr != nil {
-			return nil, s.handleSoraRequestError(ctx, account, storeErr, reqModel, c, clientStream)
+			// 存储失败时降级使用原始 URL，不中断用户请求
+			log.Printf("[Sora] StoreFromURLs failed, falling back to original URLs: %v", storeErr)
+		} else {
+			finalURLs = s.normalizeSoraMediaURLs(stored)
 		}
-		finalURLs = s.normalizeSoraMediaURLs(stored)
 	}
 
 	content := buildSoraContent(mediaType, finalURLs)