feat(TRA-245): Notification service with email and in-app delivery
- Notification model with idempotency_key dedup (unique per event+object+recipient)
- NotificationDelivery audit model (pending/sent/failed/delivered per channel)
- notify() service: creates notification idempotently, enqueues per-channel tasks
- deliver_notification_task Celery task: sends email via send_mail, marks in-app
as sent without email; marks FAILED with error_detail on exception (autoretry x3)
- Event trigger helpers: notify_course_assigned, notify_attempt_limit_reached,
notify_certificate_issued, notify_certificate_expiring (daily-reminder safe)
- send_course_due_reminders_task: periodic Celery Beat stub for due-date alerts
- REST API: list notifications (with ?unread=true filter), mark-read, mark-all-read
- Admin registrations with inline delivery audit view
- Initial migration (Notification + NotificationDelivery tables)
- Pytest test suite: idempotency, delivery dispatch, view auth/filtering
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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