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/Db.php

@@ -0,0 +1,77 @@
+<?php
+
+namespace support;
+
+use Illuminate\Database\Capsule\Manager as Capsule;
+
+class Db
+{
+    /**
+     * @var Capsule
+     */
+    protected static $capsule = null;
+
+    /**
+     * 初始化
+     */
+    public static function init(): void
+    {
+        if (self::$capsule !== null) {
+            return;
+        }
+
+        $config = config('database');
+        $connections = $config['connections'] ?? [];
+
+        self::$capsule = new Capsule();
+
+        foreach ($connections as $name => $connection) {
+            self::$capsule->addConnection($connection, $name);
+        }
+
+        self::$capsule->setAsGlobal();
+        self::$capsule->bootEloquent();
+
+        if ($default = $config['default'] ?? null) {
+            self::$capsule->getDatabaseManager()->setDefaultConnection($default);
+        }
+    }
+
+    /**
+     * 获取连接
+     */
+    public static function connection(?string $name = null): \Illuminate\Database\Connection
+    {
+        if (self::$capsule === null) {
+            self::init();
+        }
+        return self::$capsule->connection($name);
+    }
+
+    /**
+     * 获取表查询构造器
+     */
+    public static function table(string $table, ?string $connection = null): \Illuminate\Database\Query\Builder
+    {
+        return self::connection($connection)->table($table);
+    }
+
+    /**
+     * 原始SQL表达式
+     */
+    public static function raw(string $value): \Illuminate\Database\Query\Expression
+    {
+        return self::connection()->raw($value);
+    }
+
+    /**
+     * 获取 Capsule 实例
+     */
+    public static function getCapsule(): Capsule
+    {
+        if (self::$capsule === null) {
+            self::init();
+        }
+        return self::$capsule;
+    }
+}