Add tests, docs, and ruff fixes for per-user email notifications
- Fix missing timezone import in email_service.py (F821)
- Fix unused lambda arg in main.py asyncio done_callback (ARG005)
- Fix E302 blank line spacing for mark_printer_stopped_by_user
- Fix F821/UP037 forward reference in user_email_pref model
- Fix SettingsPage test for duplicate "Notifications" text
- Add backend unit tests for permissions, schemas, and templates
- Add backend integration tests for user-notifications API
- Add frontend tests for NotificationsPage
- Add user_email_pref model import to test conftest
- Update CHANGELOG and README
maziggy