feat: Permission 모델 추가 (Spatie 확장)

- Spatie\Permission\Models\Permission 확장
- tenant() belongsTo 관계 추가
- tenant_id fillable 추가
- API 권한 시스템에서 사용

🤖 Generated with [Claude Code](https://claude.ai/claude-code)

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

app/Models/Permissions/Permission.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace App\Models\Permissions;
+
+use App\Models\Tenants\Tenant;
+use Spatie\Permission\Models\Permission as SpatiePermission;
+
+class Permission extends SpatiePermission
+{
+    protected $fillable = [
+        'tenant_id',
+        'name',
+        'guard_name',
+    ];
+
+    public function tenant()
+    {
+        return $this->belongsTo(Tenant::class);
+    }
+}