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

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)

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)

backend/internal/service/subscription_maintenance_queue.go

@@ -6,12 +6,14 @@ import (
 	"sync"
 )
 
-// SubscriptionMaintenanceQueue 提供“有界队列 + 固定 worker”的后台执行器。
+// SubscriptionMaintenanceQueue 提供"有界队列 + 固定 worker"的后台执行器。
 // 用于从请求热路径触发维护动作时，避免无限 goroutine 膨胀。
 type SubscriptionMaintenanceQueue struct {
-	queue chan func()
-	wg    sync.WaitGroup
-	stop  sync.Once
+	queue  chan func()
+	wg     sync.WaitGroup
+	stop   sync.Once
+	mu     sync.RWMutex // 保护 closed 标志与 channel 操作的原子性
+	closed bool
 }
 
 func NewSubscriptionMaintenanceQueue(workerCount, queueSize int) *SubscriptionMaintenanceQueue {
@@ -48,6 +50,7 @@ func NewSubscriptionMaintenanceQueue(workerCount, queueSize int) *SubscriptionMa
 
 // TryEnqueue 尝试将任务入队。
 // 当队列已满时返回 error（调用方应该选择跳过并记录告警/限频日志）。
+// 当队列已关闭时返回 error，不会 panic。
 func (q *SubscriptionMaintenanceQueue) TryEnqueue(task func()) error {
 	if q == nil {
 		return fmt.Errorf("maintenance queue is nil")
@@ -56,6 +59,13 @@ func (q *SubscriptionMaintenanceQueue) TryEnqueue(task func()) error {
 		return fmt.Errorf("maintenance task is nil")
 	}
 
+	q.mu.RLock()
+	defer q.mu.RUnlock()
+
+	if q.closed {
+		return fmt.Errorf("maintenance queue stopped")
+	}
+
 	select {
 	case q.queue <- task:
 		return nil
@@ -69,7 +79,10 @@ func (q *SubscriptionMaintenanceQueue) Stop() {
 		return
 	}
 	q.stop.Do(func() {
+		q.mu.Lock()
+		q.closed = true
 		close(q.queue)
+		q.mu.Unlock()
 		q.wg.Wait()
 	})
 }

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