feat: 인앱 업데이트 체크 API 구현

- app_versions 테이블 마이그레이션 (시스템 레벨, tenant_id 없음)
- AppVersion 모델 (SoftDeletes)
- AppVersionService: getLatestVersion, downloadApk
- AppVersionController: GET /api/v1/app/version, GET /api/v1/app/download/{id}
- ApiKeyMiddleware 화이트리스트에 api/v1/app/* 추가
- app_releases 스토리지 디스크 추가

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/Models/AppVersion.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\SoftDeletes;
+
+class AppVersion extends Model
+{
+    use SoftDeletes;
+
+    protected $fillable = [
+        'version_code',
+        'version_name',
+        'platform',
+        'release_notes',
+        'apk_path',
+        'apk_size',
+        'apk_original_name',
+        'force_update',
+        'is_active',
+        'download_count',
+        'published_at',
+        'created_by',
+        'updated_by',
+    ];
+
+    protected $casts = [
+        'version_code' => 'integer',
+        'apk_size' => 'integer',
+        'force_update' => 'boolean',
+        'is_active' => 'boolean',
+        'download_count' => 'integer',
+        'published_at' => 'datetime',
+    ];
+}