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/tests/test_pubsub.py

@@ -100,10 +100,35 @@ async def test_multiple_concurrent_subscribers_each_get_all_events():
 
 
 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()
+    """get_event_bus() late-imports cyclone.api; if the lifespan handler
+    hasn't set ``app.state.event_bus`` (or it's been cleared), the
+    AttributeError must surface so callers can fail loudly."""
+    from cyclone.api import app
+
+    # The autouse conftest fixture sets app.state.event_bus for every test
+    # so that endpoint code can call request.app.state.event_bus. We pop
+    # the key entirely here so __getattr__ raises AttributeError rather
+    # than returning None for an explicitly-None slot.
+    state_dict = app.state._state
+    saved = state_dict.pop("event_bus", None)
+    try:
+        with pytest.raises((AttributeError, RuntimeError)):
+            get_event_bus()
+    finally:
+        if saved is not None:
+            state_dict["event_bus"] = saved
+
+
+def test_get_event_bus_returns_app_state_bus():
+    """When the lifespan handler has initialised the bus, get_event_bus()
+    returns it. Validates the happy path under the same late-import."""
+    from cyclone.api import app
+    from cyclone.pubsub import EventBus
+
+    saved = getattr(app.state, "event_bus", None)
+    bus = EventBus()
+    app.state.event_bus = bus
+    try:
+        assert get_event_bus() is bus
+    finally:
+        app.state.event_bus = saved