Make it possible to link multiple transactions to an expense

When matching bank transactions to expenses, it's currently only possible to link a single transaction to an expense. In practice, a single expense can involve multiple bank movements — for example when a supplier incorrectly charges VAT on an intra-community delivery, then refunds it later. The payment (debit) and the refund (credit) both relate to the same expense, but only the debit can be linked today. Additionally, the /bank_transactions/match endpoint rejects CREDIT transactions entirely when linking to an expense (expense_id), returning a 422 with "id does not exist" — even though the transaction exists. This means refunds or corrections related to an expense cannot be associated with it at all. Describe the solution you'd like 1. Allow multiple bank transactions (both DEBIT and CREDIT) to be linked to a single expense, similar to how multiple invoices can already be linked to a single CREDIT transaction via invoice_ids. 2. Accept CREDIT transactio…