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/api/v1/events.py

@@ -64,9 +64,9 @@ def post_event(
     problem_created = False
     if created:
         problem, problem_created = maybe_create_problem(db, event)
-        notify_event(event)
+        notify_event(event, db=db)
         if problem is not None and problem_created:
-            notify_problem(problem, event)
+            notify_problem(problem, event, db=db)
         logger.info("ingest created event_id=%s type=%s host_id=%s", event.event_id, event.type, event.host_id)
     else:
         logger.info("ingest duplicate event_id=%s", event.event_id)