Delete a recipient group

Soft-deletes the group — but only while your alert routing does not point at it. A group an alert source, a routing rule or an alert template still targets answers 409 with the list of what targets it, and stays. The foreign keys there are ON DELETE SET NULL, so deleting would not break inbound alerts from Grafana or Zabbix — it would leave them with NO recipient: the alert still arrives, still lands in triage, and wakes nobody. A template default is the same story from the other side: firing a template whose default_group_uuid no longer resolves answers 422 and the alert reaches nobody. Members are untouched either way; notification history keeps pointing at the row. Repeating the call on an already-deleted group returns 404.