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>

deploy/env.native.example

@@ -44,6 +44,10 @@ SMTP_STARTTLS=true
 SMTP_SSL=false
 SMTP_MIN_SEVERITY=high
 
+# Глобальное правило: severity ≥ NOTIFY_MIN_SEVERITY → каналы из NOTIFY_CHANNELS
+NOTIFY_MIN_SEVERITY=warning
+NOTIFY_CHANNELS=telegram,webhook,email
+
 # Статус хоста по agent.heartbeat
 SAC_HEARTBEAT_STALE_MINUTES=780