monitoring02: enable alerting and add monitoring/alertmanager CNAMEs

- Switch vmalert from blackhole mode to sending alerts to local
  Alertmanager
- Import alerttonotify service so alerts route to NATS notifications
- Add monitoring and alertmanager CNAMEs to monitoring02
- Add Caddy reverse proxy for alertmanager.home.2rjus.net

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

hosts/monitoring02/default.nix

@@ -4,5 +4,6 @@
     ../../services/grafana
     ../../services/victoriametrics
     ../../services/loki
+    ../../services/monitoring/alerttonotify.nix
   ];
 }