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/app/service/ApprovalService.php

@@ -7,7 +7,7 @@ use app\model\ApprovalNode;
 use app\model\ApprovalOrder;
 use app\model\ApprovalRecord;
 use app\model\User;
-use app\util\Db;
+use support\Db;
 use Illuminate\Database\Connection;
 
 class ApprovalService
@@ -16,8 +16,6 @@ class ApprovalService
 
     public function __construct()
     {
-        // Initialize database if not already initialized
-        Db::init();
         $this->db = Db::connection();
     }