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/turnstile_service.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 )
 
@@ -20,11 +21,15 @@ type turnstileVerifier struct {
 }
 
 func NewTurnstileVerifier() service.TurnstileVerifier {
+	sharedClient, err := httpclient.GetClient(httpclient.Options{
+		Timeout: 10 * time.Second,
+	})
+	if err != nil {
+		sharedClient = &http.Client{Timeout: 10 * time.Second}
+	}
 	return &turnstileVerifier{
-		httpClient: &http.Client{
-			Timeout: 10 * time.Second,
-		},
-		verifyURL: turnstileVerifyURL,
+		httpClient: sharedClient,
+		verifyURL:  turnstileVerifyURL,
 	}
 }