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>

services/victoriametrics/default.nix

@@ -170,15 +170,12 @@ in
     };
   };
 
-  # vmalert for alerting rules - no notifier during parallel operation
+  # vmalert for alerting rules
   services.vmalert.instances.default = {
     enable = true;
     settings = {
       "datasource.url" = "http://localhost:8428";
-      # Blackhole notifications during parallel operation to prevent duplicate alerts.
-      # Replace with notifier.url after cutover from monitoring01:
-      # "notifier.url" = [ "http://localhost:9093" ];
-      "notifier.blackhole" = true;
+      "notifier.url" = [ "http://localhost:9093" ];
       "rule" = [ ../monitoring/rules.yml ];
     };
   };
@@ -191,8 +188,11 @@ in
     reverse_proxy http://127.0.0.1:8880
   '';
 
-  # Alertmanager - same config as monitoring01 but will only receive
-  # alerts after cutover (vmalert notifier is disabled above)
+  # Alertmanager
+  services.caddy.virtualHosts."alertmanager.home.2rjus.net".extraConfig = ''
+    reverse_proxy http://127.0.0.1:9093
+  '';
+
   services.prometheus.alertmanager = {
     enable = true;
     configuration = {