Delete Experiment Group

Delete a group; its experiments are never deleted, only unassigned.

The unassign is mandatory rather than optional: the membership FK is ON DELETE RESTRICT, so the delete would fail while members remain. Both statements run in the request's single transaction — base_crud only flushes, and get_session_based_on_request_type wraps the request in managed_session, which commits once on success and rolls back on any exception — so this is atomic without explicit transaction management.