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).

tests/api/workers/test_workers_api.py

@@ -0,0 +1,171 @@
+"""Tests for the Workers panel API endpoints.
+
+Covers ``GET /api/v1/workers`` (viewer-readable, always surfaces every
+known worker) and ``POST /api/v1/workers/{name}/stop`` (admin-only,
+publishes a stop intent on the bus).
+"""
+from __future__ import annotations
+
+from typing import Any
+
+import httpx
+import pytest
+
+from decnet.bus import topics as _topics
+from decnet.bus.fake import FakeBus
+from decnet.web import worker_registry as _wr
+from decnet.web.router.workers import api_control_worker as _ctl
+from decnet.web.router.workers import api_list_workers as _list
+from decnet.web.worker_registry import KNOWN_WORKERS
+
+
+@pytest.fixture(autouse=True)
+def _reset_registry() -> None:
+    _wr.reset_registry_for_tests()
+    yield
+    _wr.reset_registry_for_tests()
+
+
+@pytest.fixture
+async def fake_bus(monkeypatch) -> FakeBus:
+    bus = FakeBus()
+    await bus.connect()
+
+    async def _stub_get_app_bus() -> FakeBus:
+        return bus
+
+    # Patch the symbol the control endpoint imported into its namespace.
+    monkeypatch.setattr(_ctl, "get_app_bus", _stub_get_app_bus)
+    yield bus
+    await bus.close()
+
+
+@pytest.mark.asyncio
+async def test_list_workers_viewer_sees_all_unknown(
+    client: httpx.AsyncClient, viewer_token: str,
+) -> None:
+    resp = await client.get(
+        "/api/v1/workers",
+        headers={"Authorization": f"Bearer {viewer_token}"},
+    )
+    assert resp.status_code == 200
+    body = resp.json()
+    names = {w["name"] for w in body["workers"]}
+    assert names == set(KNOWN_WORKERS)
+    # No heartbeats have arrived in the test harness, so every row is unknown.
+    for w in body["workers"]:
+        assert w["status"] == "unknown"
+        assert w["last_heartbeat_ts"] is None
+        assert w["seconds_since"] is None
+    assert "bus_connected" in body
+    assert isinstance(body["bus_connected"], bool)
+    # `installed` flag is always present + boolean.
+    for w in body["workers"]:
+        assert "installed" in w
+        assert isinstance(w["installed"], bool)
+
+
+@pytest.mark.asyncio
+async def test_list_workers_requires_auth(client: httpx.AsyncClient) -> None:
+    resp = await client.get("/api/v1/workers")
+    assert resp.status_code == 401
+
+
+@pytest.mark.asyncio
+async def test_list_workers_reports_bus_connected_false_when_no_bus(
+    client: httpx.AsyncClient, viewer_token: str, monkeypatch,
+) -> None:
+    async def _no_bus() -> None:
+        return None
+
+    monkeypatch.setattr(_list, "get_app_bus", _no_bus)
+    resp = await client.get(
+        "/api/v1/workers",
+        headers={"Authorization": f"Bearer {viewer_token}"},
+    )
+    assert resp.status_code == 200
+    assert resp.json()["bus_connected"] is False
+
+
+@pytest.mark.asyncio
+async def test_list_workers_reports_bus_connected_true_with_fake_bus(
+    client: httpx.AsyncClient, viewer_token: str, monkeypatch,
+) -> None:
+    bus = FakeBus()
+    await bus.connect()
+
+    async def _fake_bus() -> FakeBus:
+        return bus
+
+    monkeypatch.setattr(_list, "get_app_bus", _fake_bus)
+    try:
+        resp = await client.get(
+            "/api/v1/workers",
+            headers={"Authorization": f"Bearer {viewer_token}"},
+        )
+        assert resp.status_code == 200
+        assert resp.json()["bus_connected"] is True
+    finally:
+        await bus.close()
+
+
+@pytest.mark.asyncio
+async def test_stop_worker_admin_publishes_on_bus(
+    client: httpx.AsyncClient, auth_token: str, fake_bus: FakeBus,
+) -> None:
+    topic = _topics.system_control("mutator")
+    received: list[Any] = []
+
+    sub = fake_bus.subscribe(topic)
+    await sub.__aenter__()
+
+    async def _drain() -> None:
+        async for event in sub:
+            received.append(event)
+            return
+
+    import asyncio
+    reader = asyncio.create_task(_drain())
+    # Give the subscribe a tick so the publish lands on a live reader.
+    await asyncio.sleep(0)
+
+    try:
+        resp = await client.post(
+            "/api/v1/workers/mutator/stop",
+            headers={"Authorization": f"Bearer {auth_token}"},
+        )
+        assert resp.status_code == 202
+        body = resp.json()
+        assert body == {"accepted": True, "worker": "mutator", "action": "stop"}
+
+        await asyncio.wait_for(reader, timeout=1.0)
+        assert len(received) == 1
+        ev = received[0]
+        assert ev.topic == topic
+        assert ev.payload["action"] == _topics.WORKER_CONTROL_STOP
+        assert "requested_by" in ev.payload
+        assert "ts" in ev.payload
+    finally:
+        await sub.__aexit__(None, None, None)
+
+
+@pytest.mark.asyncio
+async def test_stop_worker_viewer_forbidden(
+    client: httpx.AsyncClient, viewer_token: str, fake_bus: FakeBus,
+) -> None:
+    resp = await client.post(
+        "/api/v1/workers/mutator/stop",
+        headers={"Authorization": f"Bearer {viewer_token}"},
+    )
+    assert resp.status_code == 403
+
+
+@pytest.mark.asyncio
+async def test_stop_worker_unknown_name_404(
+    client: httpx.AsyncClient, auth_token: str, fake_bus: FakeBus,
+) -> None:
+    resp = await client.post(
+        "/api/v1/workers/nonsense/stop",
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert resp.status_code == 404