perf(后端): 完成性能优化与连接池配置

新增 DB/Redis 连接池配置与校验，并补充单测

网关请求体大小限制与 413 处理

HTTP/req 客户端池化并调整上游连接池默认值

并发槽位改为 ZSET+Lua 与指数退避

用量统计改 SQL 聚合并新增索引迁移

计费缓存写入改工作池并补测试/基准

测试: 在 backend/ 下运行 go test ./...

backend/internal/repository/pricing_service.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 )
 
@@ -16,10 +17,14 @@ type pricingRemoteClient struct {
 }
 
 func NewPricingRemoteClient() service.PricingRemoteClient {
+	sharedClient, err := httpclient.GetClient(httpclient.Options{
+		Timeout: 30 * time.Second,
+	})
+	if err != nil {
+		sharedClient = &http.Client{Timeout: 30 * time.Second}
+	}
 	return &pricingRemoteClient{
-		httpClient: &http.Client{
-			Timeout: 30 * time.Second,
-		},
+		httpClient: sharedClient,
 	}
 }