POST /api/v1/workers/{name}/start — 202 on acceptance, 404 unknown
worker, 503 if the unit file is not installed, 502 if systemctl
returns non-zero (stderr snippet in detail, full stack logged).
Admin only.
POST /api/v1/workers/start-all — best-effort: walks the worker list
in dependency order (bus → api → data-plane), skips already-active
and uninstalled units, aggregates outcomes into
{started, already_running, failed[]}. Returns 200 even on partial
failure; the caller reads the three lists.
Both endpoints delegate to the systemd_control helper, so the attack
surface for "what gets executed" is locked to `decnet-<validated-name>
.service` at two layers (router KNOWN_WORKERS + helper regex).
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).
