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/config.py

@@ -67,6 +67,9 @@ class Settings(BaseSettings):
     smtp_ssl: bool = False
     smtp_min_severity: str = "high"
 
+    notify_min_severity: str = "warning"
+    notify_channels: str = "telegram,webhook,email"
+
     # Порог «живости» агента
     sac_heartbeat_stale_minutes: int = 780