victoriametrics: fix vmalert crash by adding notifier.blackhole

vmalert requires either a notifier URL or -notifier.blackhole when
alerting rules are present. Add blackhole flag for parallel operation.

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

services/victoriametrics/default.nix

@@ -175,9 +175,10 @@ in
     enable = true;
     settings = {
       "datasource.url" = "http://localhost:8428";
-      # Notifier disabled during parallel operation to prevent duplicate alerts
-      # Uncomment after cutover from monitoring01:
+      # 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;
       "rule" = [ ../monitoring/rules.yml ];
     };
   };