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/service/usage_cleanup_service.go

@@ -151,6 +151,9 @@ func (s *UsageCleanupService) CreateTask(ctx context.Context, filters UsageClean
 }
 
 func (s *UsageCleanupService) runOnce() {
+	if s == nil {
+		return
+	}
 	if !atomic.CompareAndSwapInt32(&s.running, 0, 1) {
 		log.Printf("[UsageCleanup] run_once skipped: already_running=true")
 		return
@@ -158,7 +161,7 @@ func (s *UsageCleanupService) runOnce() {
 	defer atomic.StoreInt32(&s.running, 0)
 
 	parent := context.Background()
-	if s != nil && s.workerCtx != nil {
+	if s.workerCtx != nil {
 		parent = s.workerCtx
 	}
 	ctx, cancel := context.WithTimeout(parent, s.taskTimeout())