Wrong payment_method on payments in OB exception / CC payoff flow

In the online banking exception flow where the client pays with a credit card, the payments record can be saved as ONLINE_BANKING while the invoice stays CC.
update_transaction uses payments.payment_method_code, so receipt completion can fail even when CFS shows a paid receipt.
Fix: set payment_method_code to match the actual rail (CC) when creating/updating that payment.

Start by tracing the online banking exception and CC payoff flow into update_transaction, then inspect where payment_method_code is assigned when the payment is created or updated. Done means the payment records the actual CC rail and receipt completion no longer fails when CFS shows a paid receipt.

Written by the indexing model from the issue text.