Cancel a fulfillment order (synchronous)

Cancels a fulfillment order as a CANCELLED transition validated against the state machine version pinned on the FO. A required free-text reason plus any free-form metadata are captured on an immutable timeline event (FulfillmentOrderTrackingEvent — the cancellation moment is previousStatus; the server-validated reason always wins over a same-named metadata key) and echoed back in the response metadata. On enter, the CANCELLED state releases the inventory reservation (RELEASE_INVENTORY); the transition, the timeline event and the stock release run in a single transaction. A fulfillment order at or after INVOICED has no declared edge to CANCELLED and is rejected with 422. accountCode and the actor (user or service account) come from the JWT.