fix(monitoring): Alertmanager must HELO with a FQDN for Mailu

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

services/monitoring/helmrelease.yaml

@@ -754,6 +754,9 @@ spec:
         smtp_smarthost: mailu-front.mailu-mailserver.svc.cluster.local:25
         smtp_from: alertmanager@bstein.dev
         smtp_require_tls: false
+        # Mailu rejects the default "HELO localhost" with 504 5.5.2; the
+        # greeting has to be a fully-qualified name.
+        smtp_hello: alerts.bstein.dev
       route:
         receiver: default
         group_wait: 30s