Update a fulfillment order status (synchronous)

Transitions the fulfillment order to a new status, validated against the state machine version pinned on the FO at creation. On a valid transition it appends an immutable timeline event (FulfillmentOrderTrackingEvent) — the status change and the timeline entry run in a single transaction. Inventory effects are not applied by this endpoint. CANCELLED and REJECTED are not allowed here: they require a reason and go through the dedicated reject/cancel endpoint. accountCode and the actor (user or service account) come from the JWT.