feat(gateway): 引入使用量记录有界 worker 池与自动扩缩容

- 新增 UsageRecordWorkerPool，支持有界队列、溢出降级策略与自动扩缩容
- 将 Gateway/OpenAI/Sora/Gemini 使用量记录改为提交到统一任务池执行
- 增加 usage_record 配置默认值与校验规则，并补充配置与任务提交相关测试
- 注入并托管 worker 池生命周期，服务退出时统一 StopAndWait

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

backend/cmd/server/wire.go

@@ -77,6 +77,7 @@ func provideCleanup(
 	pricing *service.PricingService,
 	emailQueue *service.EmailQueueService,
 	billingCache *service.BillingCacheService,
+	usageRecordWorkerPool *service.UsageRecordWorkerPool,
 	subscriptionService *service.SubscriptionService,
 	oauth *service.OAuthService,
 	openaiOAuth *service.OpenAIOAuthService,
@@ -176,6 +177,12 @@ func provideCleanup(
 				billingCache.Stop()
 				return nil
 			}},
+			{"UsageRecordWorkerPool", func() error {
+				if usageRecordWorkerPool != nil {
+					usageRecordWorkerPool.Stop()
+				}
+				return nil
+			}},
 			{"OAuthService", func() error {
 				oauth.Stop()
 				return nil