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/service/gemini_oauth_service.go

@@ -7,13 +7,13 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"net/url"
 	"strconv"
 	"strings"
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
 )
 
 type GeminiOAuthService struct {
@@ -497,11 +497,12 @@ func fetchProjectIDFromResourceManager(ctx context.Context, accessToken, proxyUR
 	req.Header.Set("Authorization", "Bearer "+accessToken)
 	req.Header.Set("User-Agent", geminicli.GeminiCLIUserAgent)
 
-	client := &http.Client{Timeout: 30 * time.Second}
-	if strings.TrimSpace(proxyURL) != "" {
-		if proxyURLParsed, err := url.Parse(strings.TrimSpace(proxyURL)); err == nil {
-			client.Transport = &http.Transport{Proxy: http.ProxyURL(proxyURLParsed)}
-		}
+	client, err := httpclient.GetClient(httpclient.Options{
+		ProxyURL: strings.TrimSpace(proxyURL),
+		Timeout:  30 * time.Second,
+	})
+	if err != nil {
+		client = &http.Client{Timeout: 30 * time.Second}
 	}
 
 	resp, err := client.Do(req)