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

@@ -0,0 +1,47 @@
+<?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);
+    }
+}