Create a reversal invoice for a fiscal invoice.
storno_bill_id.
Both invoices record a history event — the source gets a storno entry and
the new reversal gets a storned entry. The whole flow runs inside a
transaction: if any step fails, nothing is persisted.
400 with codes
invoice_cannot_be_storned or invoice_already_storned.
/invoices/{id}/storno over DELETE /invoices/{id} once an invoice has
been issued to a client or sent to SPV — deleting a fiscal invoice is
blocked by the series-continuity rules.