feat(notifications): add per-user notification system with inbox, email, and webhook targets

- Add UserNotificationTarget, UserNotificationPreference, InAppNotification models
- Add migration 025_add_user_notifications (tables + indexes)
- Add app/utils/user_notification.py dispatch service
- Add app/api/notifications.py REST endpoints (inbox, targets, preferences)
- Add app/views/notifications.py view route
- Add frontend/templates/notifications_dashboard.html Alpine.js dashboard
- Add bell icon with unread badge in base.html nav (desktop + mobile)
- Register routers in app/api/__init__.py and app/views/__init__.py
- Add 32 unit tests in tests/test_notifications_api.py

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app/api/__init__.py

@@ -20,6 +20,7 @@ from app.api.google_drive import router as google_drive_router
 from app.api.imap_accounts import router as imap_accounts_router
 from app.api.integrations import router as integrations_router
 from app.api.logs import router as logs_router
+from app.api.notifications import router as notifications_router
 from app.api.onboarding import router as onboarding_router
 from app.api.onedrive import router as onedrive_router
 from app.api.openai import router as openai_router
@@ -74,3 +75,4 @@ router.include_router(billing_router)
 router.include_router(pipelines_router)
 router.include_router(imap_accounts_router)
 router.include_router(integrations_router)
+router.include_router(notifications_router)