When app.notifications is the import entry point (e.g. Celery loading
app.notifications.service before any ORM code), app.db re-entered the
half-initialized app.notifications.persistence at its model-registration import
and failed with "cannot import name 'Notification' ... partially initialized".
Import app.db at the top of app/notifications/__init__.py so db fully initializes
(including its own Notification registration) before we re-import from
.persistence. Pre-existing issue (reproduces on the base commit); surfaced by the
celery worker startup.
