feat: SMTP email notification channel with UI and ingest

Migration 006, email config DB/env, smtplib sender, settings API,
Settings UI section, and dispatch on event/problem ingest.

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

deploy/env.native.example

@@ -32,6 +32,18 @@ WEBHOOK_SECRET_HEADER=
 WEBHOOK_SECRET=
 WEBHOOK_MIN_SEVERITY=high
 
+# SMTP email (F-NOT-01)
+SMTP_ENABLED=false
+SMTP_HOST=
+SMTP_PORT=587
+SMTP_USER=
+SMTP_PASSWORD=
+SMTP_FROM=
+SMTP_TO=
+SMTP_STARTTLS=true
+SMTP_SSL=false
+SMTP_MIN_SEVERITY=high
+
 # Статус хоста по agent.heartbeat
 SAC_HEARTBEAT_STALE_MINUTES=780