feat: global notification policy severity to channels (notif-22)

Singleton notification_policy table, NOTIFY_MIN_SEVERITY/CHANNELS env,
central dispatch gate, Settings policy UI, migration 007.

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

backend/app/models/__init__.py

@@ -2,6 +2,7 @@ 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.notification_policy import NotificationPolicy
 from app.models.problem import Problem, ProblemEvent
 
-__all__ = ["ApiKey", "Event", "Host", "NotificationChannel", "Problem", "ProblemEvent"]
+__all__ = ["ApiKey", "Event", "Host", "NotificationChannel", "NotificationPolicy", "Problem", "ProblemEvent"]