Restore a previously archived group identified by its external reference ID (apiId): its archived (HIDDEN) flag is cleared and it becomes active again. The group is restored as a root group — its previous parent link, cleared on archive, is not re-established. When several archived groups share the same external reference ID, the most recently archived one is restored; active groups are never selected. Restoring when only active groups carry the reference ID returns 409 Conflict with error code GROUP_NOT_ARCHIVED; an unknown reference ID returns 404 GROUP_NOT_FOUND. On a successful restore the GROUP_UNREMOVED webhook is dispatched.