Add "webhook" to KNOWN_WORKERS + the start-all preferred order so the
Config → Workers panel picks up the row automatically: heartbeat
subscription, start/stop controls via the existing systemd helper
(decnet-webhook.service.j2 already lands via decnet init's unit
glob), and the status-dot lifecycle all come for free.
Placed between mutator and the swarm-only agent/forwarder/updater
trio — matches the intended startup sequence (bus → api → data-plane
workers → egress → swarm management).
No frontend change needed; Config.tsx reads the worker list
dynamically from GET /api/v1/workers.
Ships the backend half of Config → Workers:
* Worker registry aggregates `system.*.health` + `system.bus.health`
heartbeats into a last-seen dict; OK / STALE / UNKNOWN tiers drop
out of a 90s window (3× the 30s heartbeat interval).
* `GET /api/v1/workers` returns the snapshot plus `bus_connected`
(so the UI can explain "all UNKNOWN" when the bus socket is down)
and a per-row `installed` flag populated from
`systemctl list-unit-files decnet-*.service` (cached 30s).
* `POST /api/v1/workers/{name}/stop` publishes a stop intent on
`system.<name>.control`; workers listen via the shared control
listener in `bus/publish.py`.
* Heartbeat + control listener wired into collector / profiler /
sniffer / prober / mutator worker loops. API self-heartbeats too
so the panel always has one ground-truth row.
* Topic helper `system_control(name)` + tests covering builder
validation, control listener shutdown path, and the API surface
(auth gating, bus-connected field, unknown-name 404).
Adds `StartFailure` / `StartAllResponse` models in anticipation of
the upcoming start endpoints (DEBT-034).
