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>

backend/app/config.py

@@ -56,6 +56,17 @@ class Settings(BaseSettings):
     webhook_secret: str = ""
     webhook_min_severity: str = "high"
 
+    smtp_enabled: bool = False
+    smtp_host: str = ""
+    smtp_port: int = 587
+    smtp_user: str = ""
+    smtp_password: str = ""
+    smtp_from: str = ""
+    smtp_to: str = ""
+    smtp_starttls: bool = True
+    smtp_ssl: bool = False
+    smtp_min_severity: str = "high"
+
     # Порог «живости» агента
     sac_heartbeat_stale_minutes: int = 780