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/web/api.py

@@ -30,6 +30,7 @@ ingestion_task: Optional[asyncio.Task[Any]] = None
 collector_task: Optional[asyncio.Task[Any]] = None
 attacker_task: Optional[asyncio.Task[Any]] = None
 sniffer_task: Optional[asyncio.Task[Any]] = None
+heartbeat_task: Optional[asyncio.Task[Any]] = None
 
 
 def get_background_tasks() -> dict[str, Optional[asyncio.Task[Any]]]:
@@ -45,6 +46,7 @@ def get_background_tasks() -> dict[str, Optional[asyncio.Task[Any]]]:
 @asynccontextmanager
 async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     global ingestion_task, collector_task, attacker_task, sniffer_task
+    global heartbeat_task
 
     import resource
     soft, hard = resource.getrlimit(resource.RLIMIT_NOFILE)
@@ -115,10 +117,33 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     else:
         log.info("Contract Test Mode: skipping background worker startup")
 
+    # Worker registry + API self-heartbeat — always on, even under
+    # contract-test mode, so the Workers panel can render the process
+    # without the dev needing to run a full stack.  A missing bus turns
+    # both into no-ops inside the helpers.
+    try:
+        from decnet.bus.app import get_app_bus
+        from decnet.bus.publish import run_health_heartbeat
+        from decnet.web.worker_registry import get_registry
+
+        _bus = await get_app_bus()
+        await get_registry().start(_bus)
+        if heartbeat_task is None or heartbeat_task.done():
+            heartbeat_task = asyncio.create_task(
+                run_health_heartbeat(_bus, "api"),
+            )
+    except Exception as exc:  # noqa: BLE001
+        log.warning("worker registry bootstrap failed: %s", exc)
+
     yield
 
     log.info("API shutdown cancelling background tasks")
-    for task in (ingestion_task, collector_task, attacker_task, sniffer_task):
+    try:
+        from decnet.web.worker_registry import get_registry
+        await get_registry().stop()
+    except Exception as exc:  # noqa: BLE001
+        log.warning("worker registry stop raised: %s", exc)
+    for task in (ingestion_task, collector_task, attacker_task, sniffer_task, heartbeat_task):
         if task and not task.done():
             task.cancel()
             try: