Cancel a distributor's cloud order and release its inventory holds. Each
sub-order still cancellable per the state machine transitions to cancelled, its
reservations are released (freeing stock), and any issued-but-unpaid invoice
is voided (a paid invoice is left intact). Scoped to the calling distributor
(404 if not owned). Response sets cache-control: no-store.
Idempotent — an already-cancelled order returns 200. If nothing is cancellable
because the order is too far along (shipped / delivered / completed), the current
status is returned with 409.