Restore a previously archived group by its internal Edusign ID: 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. Restoring a group that is already active returns 409 Conflict with error code GROUP_NOT_ARCHIVED; an unknown ID returns 404 GROUP_NOT_FOUND. On a successful restore the GROUP_UNREMOVED webhook is dispatched.