fix: [loan] 상품권 접대비 연동 시 receipt_no에 시리얼번호 매핑

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

app/Services/LoanService.php

@@ -303,6 +303,7 @@ private function syncGiftCertificateExpense(Loan $loan): void
                         'expense_date' => $loan->settlement_date ?? $loan->loan_date,
                         'amount' => $loan->amount,
                         'description' => ($metadata['cert_name'] ?? '상품권') . ' 접대비 전환',
+                        'receipt_no' => $metadata['serial_number'] ?? null,
                         'vendor_name' => $metadata['vendor_name'] ?? null,
                         'vendor_id' => ! empty($metadata['vendor_id']) ? (int) $metadata['vendor_id'] : null,
                         'payment_method' => ExpenseAccount::PAYMENT_CASH,