Cancel a duty handover

A handover that has not started yet is removed outright; one already running is ended at this instant so the record of who actually covered, and until when, survives. The outcome field says which of the two happened — that difference matters to the caller, so this endpoint answers 200 with a body rather than an empty 204. Handovers whose window has already elapsed are not cancellable and return 404: reviving one by pushing its end forward would be worse than refusing. Requires the duty-handover feature to be switched on for the deployment.