feat: implement withdrawal improvements P01-P05

- P04: fix Withdrawal.TableName() to return 'withdrawals'
- P03: add WithdrawalStatus* and WithdrawalMethod* constants; replace magic numbers; add CommissionTypeWithdrawCancel=338
- P01: add POST /v1/public/user/withdrawal_cancel to cancel pending withdrawals with ownership check, row lock, commission refund, and audit log
- P02: add method/account/qr_code_url fields to Withdrawal model and API; add input validation (qr_code_url required for Alipay/Wechat, account required for bank)
- P05: add method filter to admin GET /v1/admin/user/withdrawal/list

Co-authored-by: multica-agent <github@multica.ai>

internal/logic/common/withdrawal.go

@@ -41,7 +41,7 @@ func LoadPendingWithdrawalForUpdate(ctx context.Context, tx *gorm.DB, withdrawal
 		First(&withdrawal).Error; err != nil {
 		return nil, err
 	}
-	if withdrawal.Status != 0 {
+	if withdrawal.Status != usermodel.WithdrawalStatusPending {
 		return nil, errors.New("withdrawal status invalid")
 	}
 	return &withdrawal, nil