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/support/bootstrap.php

@@ -13,6 +13,7 @@
  */
 
 use Dotenv\Dotenv;
+use support\Db;
 use support\Log;
 use Webman\Bootstrap;
 use Webman\Config;
@@ -73,6 +74,13 @@ foreach (config('plugin', []) as $firm => $projects) {
     }
 }
 
+// 加载数据库类
+include_once __DIR__ . '/Db.php';
+include_once __DIR__ . '/Model.php';
+
+// 初始化数据库
+Db::init();
+
 Middleware::load(config('middleware', []));
 foreach (config('plugin', []) as $firm => $projects) {
     foreach ($projects as $name => $project) {