Hard-delete an invoice. Allowed only when the invoice is provably inert — a InvoiceStatus.Draft (never sent) with no payment, no dispensed line, and not the product of a merge. Anything with committed side effects is refused with 409. An invoice that came from an estimate is refused too: use "revert to estimate" so the source estimate is returned to a clean state rather than left accepted-but-unbilled. Deleting a paid/dispensed bill would strand payments and inventory — those must be reversed through their own paths first.