refactor: fix directory structure and namespaces for webman conventions

- Rename directories to lowercase: Controller→controller, Model→model, Service→service, Util→util, Bootstrap→bootstrap
- Update all namespaces from App\Xxx to app\xxx (webman convention)
- Update all import statements across the codebase
- Update route.php to use lowercase namespaces
- Remove unused CompanyFeishuApp model
- Comment out unimplemented controller routes

Following webman 2.x framework conventions:
- Models: app\model\{ClassName}
- Controllers: app\controller\{ClassName}
- Services: app\service\{ClassName}
- Utils: app\util\{ClassName}
- Middleware: app\middleware\{ClassName}

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

backend/app/Model/ApprovalFlow.php

@@ -1,47 +0,0 @@
-<?php
-
-namespace App\Model;
-
-use support\Model;
-
-class ApprovalFlow extends Model
-{
-    protected $table = 'approval_flow';
-
-    protected $fillable = [
-        'business_type',
-        'name',
-        'description',
-        'status',
-    ];
-
-    protected $casts = [
-        'status' => 'integer',
-        'created_at' => 'datetime',
-        'updated_at' => 'datetime',
-    ];
-
-    /**
-     * 关联审批节点
-     */
-    public function nodes()
-    {
-        return $this->hasMany(ApprovalNode::class, 'flow_id')->orderBy('order');
-    }
-
-    /**
-     * 获取启用的流程
-     */
-    public function scopeActive($query)
-    {
-        return $query->where('status', 1);
-    }
-
-    /**
-     * 根据业务类型获取流程
-     */
-    public function scopeByBusinessType($query, string $businessType)
-    {
-        return $query->where('business_type', $businessType);
-    }
-}