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_messages_compat_service.go

@@ -921,7 +921,10 @@ func sleepGeminiBackoff(attempt int) {
 	time.Sleep(sleepFor)
 }
 
-var sensitiveQueryParamRegex = regexp.MustCompile(`(?i)([?&](?:key|client_secret|access_token|refresh_token)=)[^&"\s]+`)
+var (
+	sensitiveQueryParamRegex = regexp.MustCompile(`(?i)([?&](?:key|client_secret|access_token|refresh_token)=)[^&"\s]+`)
+	retryInRegex             = regexp.MustCompile(`Please retry in ([0-9.]+)s`)
+)
 
 func sanitizeUpstreamErrorMessage(msg string) string {
 	if msg == "" {
@@ -1925,7 +1928,6 @@ func ParseGeminiRateLimitResetTime(body []byte) *int64 {
 	}
 
 	// Match "Please retry in Xs"
-	retryInRegex := regexp.MustCompile(`Please retry in ([0-9.]+)s`)
 	matches := retryInRegex.FindStringSubmatch(string(body))
 	if len(matches) == 2 {
 		if dur, err := time.ParseDuration(matches[1] + "s"); err == nil {