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/Services/PurchaseService.php

@@ -93,6 +93,7 @@ public function store(array $data): Purchase
             $purchase->supply_amount = $data['supply_amount'];
             $purchase->tax_amount = $data['tax_amount'];
             $purchase->total_amount = $data['total_amount'];
+            $purchase->purchase_type = $data['purchase_type'] ?? null;
             $purchase->description = $data['description'] ?? null;
             $purchase->status = 'draft';
             $purchase->withdrawal_id = $data['withdrawal_id'] ?? null;
@@ -147,6 +148,9 @@ public function update(int $id, array $data): Purchase
             if (array_key_exists('withdrawal_id', $data)) {
                 $purchase->withdrawal_id = $data['withdrawal_id'];
             }
+            if (array_key_exists('purchase_type', $data)) {
+                $purchase->purchase_type = $data['purchase_type'];
+            }
             if (array_key_exists('tax_invoice_received', $data)) {
                 $purchase->tax_invoice_received = $data['tax_invoice_received'];
             }