Mark Invoice as Paid

Mark an invoice as paid, allowing partial payments.

    This endpoint supports multiple payment methods:
    - Apply existing transactions as payments
    - Apply credit memos to reduce the invoice balance
    - Create manual payments without a transaction

    The request body should contain at least one of:
    - transactions: List of transaction payments to apply
    - credit_memos: List of credit memos to apply
    - empty_transactions: List of manual payments without transactions