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

backend/tests/test_pubsub.py

@@ -12,7 +12,7 @@ import time
 
 import pytest
 
-from cyclone.pubsub import EventBus
+from cyclone.pubsub import EventBus, get_event_bus
 
 
 @pytest.mark.asyncio
@@ -97,3 +97,13 @@ async def test_multiple_concurrent_subscribers_each_get_all_events():
         for _ in range(5):
             seen.append(await asyncio.wait_for(q.__anext__(), timeout=0.5))
         assert [e["i"] for e in seen] == [0, 1, 2, 3, 4]
+
+
+def test_get_event_bus_raises_when_app_state_uninitialized():
+    """get_event_bus() late-imports cyclone.api; without a running app,
+    accessing app.state.event_bus raises AttributeError."""
+    # Don't import cyclone.api at module level — that would force a load
+    # and could mask the real failure. Just call the helper and expect
+    # the AttributeError to surface from app.state.
+    with pytest.raises((AttributeError, RuntimeError)):
+        get_event_bus()