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/repository/openai_oauth_service_test.go

@@ -244,6 +244,13 @@ func (s *OpenAIOAuthServiceSuite) TestRefreshToken_NonSuccessStatus() {
 	require.ErrorContains(s.T(), err, "status 401")
 }
 
+func TestNewOpenAIOAuthClient_DefaultTokenURL(t *testing.T) {
+	client := NewOpenAIOAuthClient()
+	svc, ok := client.(*openaiOAuthService)
+	require.True(t, ok)
+	require.Equal(t, openai.TokenURL, svc.tokenURL)
+}
+
 func TestOpenAIOAuthServiceSuite(t *testing.T) {
 	suite.Run(t, new(OpenAIOAuthServiceSuite))
 }