fix(notifications): break db <-> notifications.persistence circular import

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.

surfsense_backend/app/notifications/__init__.py

@@ -6,6 +6,10 @@ the router in :mod:`app.notifications.api` exposes the inbox endpoints.
 
 from __future__ import annotations
 
+# Initialize app.db first to avoid a partial-init circular import when this
+# package is the entry point (e.g. Celery loading it before any ORM code).
+import app.db  # noqa: F401
+
 from app.notifications.persistence import Notification
 from app.notifications.service import NotificationService