refactor: 使用官方support\Db和support\Model

- 创建 support/Db.php 和 support/Model.php
- 在 bootstrap.php 中自动初始化数据库
- 移除 app/util/Db.php
- 更新所有模型和控制器使用官方类

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/composer.json

@@ -19,7 +19,8 @@
   },
   "autoload": {
     "psr-4": {
-      "app\\": "app/"
+      "app\\": "app/",
+      "support\\": "support/"
     }
   },
   "scripts": {