feat(pubsub): get_event_bus() late-import accessor

backend/src/cyclone/pubsub.py

@@ -56,3 +56,15 @@ class EventBus:
         while True:
             yield await queue.get()
             queue.task_done()
+
+
+def get_event_bus() -> EventBus:
+    """Return the process-wide EventBus attached to the FastAPI app state.
+
+    Late-imports ``cyclone.api`` to avoid a circular import at module
+    load time (the API module imports pubsub indirectly via lifespan).
+    Tests stub this via monkeypatch.
+    """
+    from cyclone.api import app  # late import to avoid circular
+
+    return app.state.event_bus