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.
Ingester connects the bus at startup, emits a batch-committed summary
(component/flushed/position) after each successful _flush_batch. Zero-
row flushes are suppressed so the topic stays meaningful.
Complements the collector's per-line system.log publishes: collector
signals ingress, ingester signals DB-persisted progress. Federation
forwarder (worker 8) will subscribe to the batch-committed leaf to
trigger its upstream push.
Bus stays optional: publish_safely swallows failures, get_bus() can
return None, DECNET_BUS_ENABLED=false leaves the ingestion loop fully
functional.
