feat(认证): 启用 OpenAI OAuth HTTP/2 并修复清理任务 lint

为共享 req 客户端增加 HTTP/2 选项与缓存隔离
OpenAI OAuth 超时提升到 120s，并按协议控制强制
新增客户端池与 OAuth 客户端单测覆盖
修复 usage cleanup 相关 errcheck/ineffassign/staticcheck 并统一格式

测试: make test

backend/internal/handler/admin/usage_cleanup_handler_test.go

@@ -3,8 +3,8 @@ package admin
 import (
 	"bytes"
 	"context"
-	"encoding/json"
 	"database/sql"
+	"encoding/json"
 	"errors"
 	"net/http"
 	"net/http/httptest"