feat(api): parse endpoints pass EventBus into store writes

- api.parse_837 / parse_835: pass request.app.state.event_bus into store.add()
- conftest: autouse fixture wires a fresh EventBus onto app.state for every
  test, since TestClient does not invoke the FastAPI lifespan handler
  unless used as a context manager
- test_pubsub: split get_event_bus coverage into a raises-when-missing test
  and a returns-attached-bus test, both save/restore app.state.event_bus
  around the assertion so the autouse fixture's bus is preserved

Phase 2 complete: db init moved to lifespan, EventBus is the process-wide
publish point, and the two ingest endpoints publish claim_written /
remittance_written / activity_recorded events on every store.add().

backend/src/cyclone/api.py

@@ -279,7 +279,7 @@ async def parse_837(
         parsed_at=utcnow(),
         result=result,
     )
-    store.add(rec)
+    store.add(rec, event_bus=request.app.state.event_bus)
 
     if _client_wants_json(request):
         body = json.loads(result.model_dump_json())
@@ -487,7 +487,7 @@ async def parse_835_endpoint(
         parsed_at=utcnow(),
         result=result,
     )
-    store.add(rec)
+    store.add(rec, event_bus=request.app.state.event_bus)
 
     if _client_wants_json(request):
         body = json.loads(result.model_dump_json())