feat(wallet): 대출 월 상환금(loan_payment) 필드 추가

- 원리금균등 상환표를 은행 조회액과 정확히 맞추기 위한 고정 월 상환금 선택 저장
- wallet.loan_payment 컬럼(ALTER IF NOT EXISTS), 도메인·DTO·매퍼 반영

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/java/com/sb/web/account/service/AccountService.java

@@ -659,6 +659,7 @@ public class AccountService {
                 .loanMethod(isLoan ? blankToNull(req.getLoanMethod()) : null)
                 .loanMonths(isLoan ? req.getLoanMonths() : null)
                 .loanStart(isLoan ? req.getLoanStart() : null)
+                .loanPayment(isLoan ? req.getLoanPayment() : null)
                 .build();
     }