fix(mcp): get_anomaly shows Steward's own restarts beside every anomaly, and a router's containers (#5862)

All 16 Traefik 4xx anomalies on 2026-10-15 came back with nothing
around them, though 15 were Steward's own restarts. A router has no
host, and every events source was read by host.

- Core: each break of more than 90 s in Steward's once-a-minute health
  sample is a "steward" event beside every anomaly, placed or not.
- Plugins: an optional mcp_resource_events(db, source, resource, since,
  until) for a resource with no host. Traefik's gives the container
  events of the swarm service behind a router, on any host.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/mcp/test_anomaly_review.py

@@ -103,3 +103,27 @@ def test_a_plugin_adds_events_through_its_hook(app):
         assert "events_test" not in mcp.event_sources
     finally:
         mcp.event_sources.pop("events_test", None)
+
+
+def test_a_plugin_adds_a_hostless_resources_events_through_its_hook(app):
+    async def events(db, source, resource, since, until):
+        return []
+
+    mcp = app.mcp_server
+    mcp.register_plugin("events_test", NS(mcp_resource_events=events))
+    try:
+        assert mcp.resource_event_sources["events_test"] is events
+        mcp.register_plugin("events_test", NS())
+        assert "events_test" not in mcp.resource_event_sources
+    finally:
+        mcp.resource_event_sources.pop("events_test", None)
+
+
+def test_steward_not_recording_is_an_event_beside_every_anomaly():
+    """A Steward restart between two of its once-a-minute samples (#5862)."""
+    ticks = [NOW + timedelta(minutes=m) for m in (0, 1, 2)] + [NOW + timedelta(minutes=4, seconds=10)]
+    [gap] = _events.gaps_in(ticks, NOW, NOW + timedelta(hours=1))
+    assert gap["kind"] == "steward" and gap["at"] == "2026-10-15T12:02:00Z"
+    assert gap["until"] == "2026-10-15T12:04:10Z" and "wasn't recording" in gap["what"]
+    assert _events.gaps_in(ticks, NOW + timedelta(minutes=5), NOW + timedelta(hours=1)) == []
+    assert _events.gaps_in([NOW + timedelta(minutes=m) for m in range(5)], NOW, NOW + timedelta(hours=1)) == []