feat: [loan] 상품권 접대비 자동 연동 기능 추가

- ExpenseAccount: loan_id 필드 + SUB_TYPE_GIFT_CERTIFICATE 상수 추가
- LoanService: 상품권 used+접대비해당 시 expense_accounts 자동 upsert/삭제
- 마이그레이션: expense_accounts에 loan_id 컬럼 추가

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

database/migrations/2026_03_05_210000_add_loan_id_to_expense_accounts_table.php

@@ -0,0 +1,32 @@
+<?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('expense_accounts', function (Blueprint $table) {
+            $table->unsignedBigInteger('loan_id')->nullable()->after('card_no')
+                ->comment('연결된 가지급금 ID (상품권→접대비 전환 시)');
+
+            $table->index('loan_id');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('expense_accounts', function (Blueprint $table) {
+            $table->dropIndex(['loan_id']);
+            $table->dropColumn('loan_id');
+        });
+    }
+};