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>

steward/mcp/server.py

@@ -82,6 +82,9 @@ class StewardMCPServer(MCPServer):
         # plugin name -> its `mcp_events(db, host_id, host_name, since, until)`:
         # what it knows happened on a host, for get_anomaly.
         self.event_sources: dict[str, object] = {}
+        # plugin name -> its `mcp_resource_events(db, source, resource, since,
+        # until)`: what it knows happened to a resource with no host.
+        self.resource_event_sources: dict[str, object] = {}
 
     # ── registration ─────────────────────────────────────────────────────────
     def register_tool(self, fn, *, role: UserRole, read_only: bool, plugin: str | None = None) -> None:
@@ -108,7 +111,9 @@ class StewardMCPServer(MCPServer):
         """Add a plugin's tools via its optional `register_mcp_tools(register)`,
         its optional `mcp_overview(db)` as a get_overview findings source, and
         its optional `mcp_events(db, host_id, host_name, since, until)` as a
-        source of what happened on a host (get_anomaly).
+        source of what happened on a host (get_anomaly), and its optional
+        `mcp_resource_events(db, source, resource, since, until)` for what
+        happened to a resource with no host.
 
         Idempotent per plugin (hot-reload drops its previous tools first) and
         tolerant: a raising hook is logged and leaves the plugin with no tools,
@@ -123,6 +128,10 @@ class StewardMCPServer(MCPServer):
         events = getattr(module, "mcp_events", None)
         if callable(events):
             self.event_sources[name] = events
+        self.resource_event_sources.pop(name, None)
+        resource_events = getattr(module, "mcp_resource_events", None)
+        if callable(resource_events):
+            self.resource_event_sources[name] = resource_events
         hook = getattr(module, "register_mcp_tools", None)
         if hook is None:
             return