feat: 实现默认审批人和抄送人功能

- 后端新增 getDefaultApprovers 接口，从审批流程配置中获取默认审批人和抄送人
- 前端申请页面加载时自动获取并显示默认审批人和抄送人
- 添加默认审批流程配置：1个审批人(闫继光) + 2个抄送人(孟启、程丽敏)
- 移除后端 support 目录下的自定义 Db 和 Model 类，使用 Webman 官方实现
- 前端选择页面优化：优先从 sessionStorage 恢复，否则加载后端默认配置

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

backend/support/Bootstrap.php

@@ -1,16 +0,0 @@
-<?php
-
-namespace support;
-
-use Webman\App;
-use Workerman\Worker;
-
-class Bootstrap
-{
-    public static function start(?Worker $worker): void
-    {
-        // 应用启动时的初始化
-        error_reporting(E_ALL);
-        ini_set('display_errors', 'on');
-    }
-}

backend/support/Db.php

@@ -1,77 +0,0 @@
-<?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;
-    }
-}

backend/support/Model.php

@@ -1,16 +0,0 @@
-<?php
-
-namespace support;
-
-use Illuminate\Database\Eloquent\Model as EloquentModel;
-
-class Model extends EloquentModel
-{
-    /**
-     * 获取数据库连接
-     */
-    public function getConnection()
-    {
-        return Db::connection($this->getConnectionName());
-    }
-}