feat(agent,forwarder,updater): publish system.<worker>.health heartbeats (DEBT-031 workers 7-9)

All three workers now share a run_health_heartbeat helper in
decnet.bus.publish.  Each publishes system.<worker>.health on a 30s tick
with {worker, ts} plus optional per-worker extras.  Subscribers can
watch system.*.health to see every DECNET worker on a host at once.

- agent: heartbeat runs inside the FastAPI lifespan alongside the
  existing master-facing heartbeat; bus-disabled path is a no-op.
- forwarder: heartbeat task spawned at run_forwarder entry, cancelled
  in the finally block so a crashed master loop never leaks the task.
- updater: new FastAPI lifespan hosts the heartbeat.

Heartbeat helper swallows extra() failures and is cancellation-safe so
lifespan teardown never hangs on it.

decnet/updater/app.py

@@ -9,24 +9,67 @@ only — agent certs are rejected).
 """
 from __future__ import annotations
 
+import asyncio
+import contextlib
 import os as _os
 import pathlib
+from contextlib import asynccontextmanager
+from typing import Optional
 
 from fastapi import FastAPI, File, Form, HTTPException, UploadFile
 from pydantic import BaseModel
 
+from decnet.bus.factory import get_bus
+from decnet.bus.publish import run_health_heartbeat
 from decnet.logging import get_logger
 from decnet.swarm import pki
 from decnet.updater import executor as _exec
 
 log = get_logger("updater.app")
 
+
+_bus_heartbeat_task: Optional[asyncio.Task] = None
+
+
+@asynccontextmanager
+async def _lifespan(_app: FastAPI):
+    # Host-local bus heartbeat (system.updater.health).  Lets the agent
+    # and dashboard tell "updater's up" without hitting the HTTPS port.
+    # Bus-disabled path is a no-op loop; the updater serves requests
+    # either way.
+    bus = None
+    try:
+        bus = get_bus(client_name="updater")
+        await bus.connect()
+    except Exception as exc:  # noqa: BLE001
+        log.warning("updater: bus unavailable, skipping heartbeat: %s", exc)
+        bus = None
+
+    global _bus_heartbeat_task
+    _bus_heartbeat_task = asyncio.create_task(
+        run_health_heartbeat(bus, "updater"),
+        name="updater-bus-heartbeat",
+    )
+    try:
+        yield
+    finally:
+        if _bus_heartbeat_task is not None:
+            _bus_heartbeat_task.cancel()
+            with contextlib.suppress(asyncio.CancelledError, Exception):
+                await _bus_heartbeat_task
+            _bus_heartbeat_task = None
+        if bus is not None:
+            with contextlib.suppress(Exception):
+                await bus.close()
+
+
 app = FastAPI(
     title="DECNET Self-Updater",
     version="0.1.0",
     docs_url=None,
     redoc_url=None,
     openapi_url=None,
+    lifespan=_lifespan,
 )