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>

database/migrations/2025_12_27_164149_add_purchase_type_to_purchases_table.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('purchases', function (Blueprint $table) {
+            $table->string('purchase_type', 50)->nullable()->after('status')->comment('매입유형');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('purchases', function (Blueprint $table) {
+            $table->dropColumn('purchase_type');
+        });
+    }
+};