feat: test-alert subcommand + enable email for bredelar.info

- Add `test-alert [--level yellow|red]` to verify SMTP without a real scan
- Enable email alerting in bredelar.info/config.yaml (uses DEFAULT_CONFIG
  for host/user/from/to, only smtp_password_env kept explicit)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bredelar.info/config.yaml

@@ -75,15 +75,8 @@ periodic_checks:
 alerting:
   min_level: "yellow"
   email:
-    enabled: false
-    smtp_host: "localhost"
-    smtp_port: 587
-    smtp_tls: true
-    smtp_user: ""
+    enabled: true
     smtp_password_env: "SCANNER_SMTP_PASSWORD"
-    from: "scanner@bredelar.info"
-    to:
-      - "admin@bredelar.info"
   webhook:
     enabled: false
     url: ""