Delete a recipient

Soft-deletes the recipient — but only while nothing holds them. Someone who belongs to a group, has a current or upcoming shift or handover, is named in an escalation step, or is a template default answers 409 with the list of what holds them, and stays. That refusal matters most for duty: the delete is soft, so their shifts survive in the database while the link goes under the soft-delete scope, and the schedule starts answering «nobody is on call» — data intact, duty gone. Past shifts do not hold: history is not a reference. Notification history keeps pointing at the row either way. Repeating the call on an already-deleted recipient returns 404.