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/Http/Requests/V1/Purchase/StorePurchaseRequest.php

@@ -19,6 +19,7 @@ public function rules(): array
             'supply_amount' => ['required', 'numeric', 'min:0'],
             'tax_amount' => ['required', 'numeric', 'min:0'],
             'total_amount' => ['required', 'numeric', 'min:0'],
+            'purchase_type' => ['nullable', 'string', 'max:50'],
             'description' => ['nullable', 'string', 'max:1000'],
             'withdrawal_id' => ['nullable', 'integer', 'exists:withdrawals,id'],
         ];