refactor: simplify mqtt pipeline state and remove event table

backend/tests/mqtt/test_models.py

@@ -18,4 +18,5 @@ def test_terminal_message_raw_model_can_persist() -> None:
         session.refresh(row)
 
     assert row.id is not None
-    assert row.parse_status == "pending"
+    assert row.parsed_at is None
+    assert row.parse_error is None