Deletes a draft invoice, discarding it completely.

Only an invoice that was never finalized can be deleted — a draft, or a generation attempt that failed. The invoice disappears from the payment provider as well, and its billing period returns to Open so it can be invoiced again. A finalized invoice must be voided through POST /invoice/{invoiceId}/void instead; deleting one responds 409 Conflict, as does deleting while generation is still in progress.