Delete an on-call schedule

Hard delete: every shift and handover of this schedule goes with it. There is no undo and no soft-delete. A schedule that an escalation step still points at answers 409 with the list of what points at it and stays — the reference lives as a plain string inside the step JSON, with no foreign key to protect it, so deleting would leave that step resolving nobody. Steps count wherever they live: in a policy, or inline in a group notification setting. To take a rotation out of use without losing its history, PATCH is_active to false instead. Repeating the call returns 404.