feat: route internal events through a shared plugin event bus

Formalises three ad-hoc publish paths that already existed:
automation-events.ts, metrics/automation-bridge.ts and the
hostSessionStatus singleton, now the host.session.status topic.

Both invariants the old code relied on are preserved. Publishing stays
fire-and-forget, so a failing subscriber cannot disturb a metrics poll or
a host delete. The automations engine subscribes to the bus at scheduler
start instead of being imported ad hoc, which keeps the edge
one-directional: automations may import repositories, hosts modules must
not import automations.

src/backend/hosts/host-session-status.ts

@@ -1,5 +1,12 @@
+import { pluginEvents, TOPICS } from "../plugins/events.js";
+
 type HostSessionStatusListener = (hostId: number, online: boolean) => void;
 
+export interface HostSessionStatusPayload {
+  hostId: number;
+  online: boolean;
+}
+
 export class HostSessionStatus {
   private counts = new Map<number, number>();
   private listeners = new Set<HostSessionStatusListener>();
@@ -32,6 +39,12 @@ export class HostSessionStatus {
 
   private emit(hostId: number, online: boolean): void {
     for (const listener of this.listeners) listener(hostId, online);
+    // Also published as a topic so plugins (and anything else on the bus) can
+    // observe session status without holding a reference to this singleton.
+    pluginEvents.emit(TOPICS.hostSessionStatus, {
+      hostId,
+      online,
+    } satisfies HostSessionStatusPayload);
   }
 }