Thin async wrapper over `systemctl` — never shell=True, always
create_subprocess_exec. Unit names are built from
`decnet-<validated-name>.service`; the regex check is defence in depth
on top of the router-level KNOWN_WORKERS validation.
Exposes start / stop / is_active / list_installed; last is cached for
30s to keep the Workers panel cheap under REFRESH spam. On non-systemd
hosts list_installed returns an empty set, so the UI renders with
every row marked not-installed instead of 500-ing.
