monitoring02: enable alerting and migrate CNAMEs from http-proxy

- Switch vmalert from blackhole mode to sending alerts to local
  Alertmanager
- Import alerttonotify service so alerts route to NATS notifications
- Move alertmanager and grafana CNAMEs from http-proxy to monitoring02
- Add monitoring CNAME to monitoring02
- Add Caddy reverse proxy entries for alertmanager and grafana
- Remove prometheus, alertmanager, and grafana Caddy entries from
  http-proxy (now served directly by monitoring02)
- Add shared/nats/nkey to monitoring02 Vault AppRole policy

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
   ];
 }