feat: 매입관리 품의서/지출결의서 연동 기능 추가

- purchases 테이블에 approval_id 컬럼 추가 (마이그레이션)
- Purchase 모델에 approval 관계 정의
- PurchaseService에서 approval 데이터 eager loading 구현
- FormRequest에 approval_id 유효성 검증 추가
- Swagger 문서에 approval 관련 스키마 추가

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

database/migrations/2025_01_22_100000_add_approval_id_to_purchases_table.php

@@ -0,0 +1,34 @@
+<?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->unsignedBigInteger('approval_id')
+                ->nullable()
+                ->after('withdrawal_id')
+                ->comment('연결된 품의서/지출결의서 ID');
+
+            $table->index('approval_id', 'idx_approval');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('purchases', function (Blueprint $table) {
+            $table->dropIndex('idx_approval');
+            $table->dropColumn('approval_id');
+        });
+    }
+};