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/internal/handler/sora_gateway_handler_test.go

@@ -432,7 +432,7 @@ func TestSoraGatewayHandler_ChatCompletions(t *testing.T) {
 	soraClient := &stubSoraClient{imageURLs: []string{"https://example.com/a.png"}}
 	soraGatewayService := service.NewSoraGatewayService(soraClient, nil, nil, cfg)
 
-	handler := NewSoraGatewayHandler(gatewayService, soraGatewayService, concurrencyService, billingCacheService, cfg)
+	handler := NewSoraGatewayHandler(gatewayService, soraGatewayService, concurrencyService, billingCacheService, nil, cfg)
 
 	rec := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(rec)