Ports the v1 DatabaseNotifications routes to the Huma /api/v2 API:
- GET /notifications lists the caller's own notifications (paginated)
- PUT /notifications/{notificationid} marks one (un-)read
- POST /notifications is a custom action marking all as read; the
link-share guard, session and commit live in the handler since there
is no CRUDable Do* for a bulk mark.
Adds fixture rows and a webtest matrix mirroring the v1 model behaviour
(own-only visibility, mark-(un)read, link-share refusal on every route).
kolaente