Closes a billing period, writing off whatever it would still have invoiced.

The remaining amount is covered by a write-off voucher carrying the caller and the reason, so the period ends up closed and fully covered; nothing is sent to the payment provider. Answers 409 Conflict for a period that has already been invoiced, is being reconciled, or was already closed this way. The close is sticky: later changes to the period's billing inputs no longer bring it back for review, and undoing it means calling POST /billing-period/{billingPeriodId}/resettle with reopen set.