Flag for Pending Invoice Cancellation/Crediting.

When there is a pending pro rata change (such as an upgrade or downgrade), a previous invoice might be credited if the new one is paid and the pro rata change is enacted.

This introduces a new flag on invoices, indicating pending credits due to pro-rata adjustments. When actions like upgrades or downgrades occur, resulting in potential credits, this flag, named "to_be_credited," on affected invoices.