perf(网关): 实现上游账号连接池隔离

新增隔离策略与连接池缓存回收

连接池大小跟随账号并发并处理代理切换

同步配置默认值与示例并补充测试

backend/internal/service/openai_gateway_service.go

@@ -311,7 +311,7 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 	}
 
 	// Send request
-	resp, err := s.httpUpstream.Do(upstreamReq, proxyURL)
+	resp, err := s.httpUpstream.Do(upstreamReq, proxyURL, account.ID, account.Concurrency)
 	if err != nil {
 		return nil, fmt.Errorf("upstream request failed: %w", err)
 	}