feat(workers): bus-backed Workers panel (registry, control, installed flag)

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).

decnet/prober/worker.py

@@ -30,7 +30,11 @@ from typing import Any, Callable
 from decnet.bus import topics as _topics
 from decnet.bus.base import BaseBus
 from decnet.bus.factory import get_bus
-from decnet.bus.publish import make_thread_safe_publisher
+from decnet.bus.publish import (
+    make_thread_safe_publisher,
+    run_control_listener,
+    run_health_heartbeat,
+)
 from decnet.logging import get_logger
 from decnet.prober.hassh import hassh_server
 from decnet.prober.jarm import JARM_EMPTY_HASH, jarm_hash
@@ -519,8 +523,13 @@ async def prober_worker(
             event_type,
         )
 
+    shutdown = asyncio.Event()
+    heartbeat_task = asyncio.create_task(run_health_heartbeat(bus, "prober"))
+    control_task = asyncio.create_task(
+        run_control_listener(bus, "prober", shutdown),
+    )
     try:
-        while True:
+        while not shutdown.is_set():
             # Discover new attacker IPs from the log stream
             new_ips, log_position = await asyncio.to_thread(
                 _discover_attackers, json_path, log_position,
@@ -542,8 +551,15 @@ async def prober_worker(
                     _publish_attacker,
                 )
 
-            await asyncio.sleep(interval)
+            try:
+                await asyncio.wait_for(shutdown.wait(), timeout=interval)
+            except asyncio.TimeoutError:
+                pass
     finally:
+        for t in (heartbeat_task, control_task):
+            t.cancel()
+            with contextlib.suppress(Exception, asyncio.CancelledError):
+                await t
         if bus is not None:
             with contextlib.suppress(Exception):
                 await bus.close()