feat: 매입유형(purchase_type) 필드 추가

- 마이그레이션: purchases 테이블에 purchase_type 컬럼 추가
- Purchase 모델: $fillable 및 PURCHASE_TYPES 상수 추가
- StorePurchaseRequest: purchase_type validation 추가
- UpdatePurchaseRequest: purchase_type validation 추가
- PurchaseService: store/update에 purchase_type 처리 추가

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

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

app/Models/Tenants/Purchase.php

@@ -21,6 +21,7 @@ class Purchase extends Model
         'total_amount',
         'description',
         'status',
+        'purchase_type',
         'withdrawal_id',
         'tax_invoice_received',
         'created_by',
@@ -28,6 +29,28 @@ class Purchase extends Model
         'deleted_by',
     ];
 
+    /**
+     * 매입 유형 목록
+     */
+    public const PURCHASE_TYPES = [
+        'unset' => '미설정',
+        'raw_material' => '원재료매입',
+        'subsidiary_material' => '부재료매입',
+        'product' => '상품매입',
+        'outsourcing' => '외주가공비',
+        'consumables' => '소모품비',
+        'repair' => '수선비',
+        'transportation' => '운반비',
+        'office_supplies' => '사무용품비',
+        'rent' => '임차료',
+        'utilities' => '수도광열비',
+        'communication' => '통신비',
+        'vehicle' => '차량유지비',
+        'entertainment' => '접대비',
+        'insurance' => '보험료',
+        'other_service' => '기타용역비',
+    ];
+
     protected $casts = [
         'purchase_date' => 'date',
         'supply_amount' => 'decimal:2',