feat: Telegram settings in DB with UI edit and test send

Store notification_channels (migration 004), effective config DB over env,
PUT/test API endpoints, Settings form, and pass db session into ingest notify.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/__init__.py

@@ -1,6 +1,7 @@
 from app.models.api_key import ApiKey
 from app.models.event import Event
 from app.models.host import Host
+from app.models.notification_channel import NotificationChannel
 from app.models.problem import Problem, ProblemEvent
 
-__all__ = ["ApiKey", "Event", "Host", "Problem", "ProblemEvent"]
+__all__ = ["ApiKey", "Event", "Host", "NotificationChannel", "Problem", "ProblemEvent"]