feat: 交付官方 Docker 运行时，外观改背景图并下线自定义 CSS/JS

站点/后台分轨背景与用户列表排序一并落地；生产 CORS 改走 SITE_URL，健康检查带版本号。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/service/auth.go

@@ -98,12 +98,21 @@ func (s *AuthService) Register(username, email, password string) (*model.User, e
 		return nil, err
 	}
 
+	var n int64
+	if err := s.db.Model(&model.User{}).Count(&n).Error; err != nil {
+		return nil, err
+	}
+	role := model.RoleUser
+	if n == 0 {
+		role = model.RoleOwner
+	}
+
 	user := &model.User{
 		Username: username,
 		Email:    email,
 		Password: string(hashed),
 		Nickname: username,
-		Role:     model.RoleUser,
+		Role:     role,
 	}
 	if err := s.db.Create(user).Error; err != nil {
 		return nil, err