fix(ui): Standby-Node zeigt keinen "alle Backends down"-Fehlalarm mehr — v1.3.8

Der Dashboard-Down-Backends-Alarm liest die LIVE-HAProxy-Stats des lokalen
Nodes. Auf dem keepalived-BACKUP-Node erreicht die lokale HAProxy die Backend-
Subnetze nicht (VLAN-Gateway-VIPs liegen beim Master) → alle Backends L4-down.
v1.3.7 stoppte nur den alert_events-Spam (scheduler), nicht die Anzeige.

Jetzt: ist der Node BACKUP (vip_status.vrrp_state), wird der rote "N Backends
down"-Alarm durch einen ruhigen Info-Hinweis ersetzt ("Standby-Node — Backend-
Health lokal nicht aussagekräftig, Master bedient den Traffic"). Auf MASTER/
UNKNOWN bleibt der echte Alarm.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

management-ui/src/i18n/locales/en/common.json

@@ -454,6 +454,8 @@
       "summaryCritical": "{{critical}} critical"
     },
     "downBackendsAlert": "{{count}} backend(s) completely down — no server UP",
+    "backupNodeBackends": "Standby node — local backend health is not meaningful",
+    "backupNodeBackendsDesc": "This node is currently the keepalived BACKUP and does not hold the VLAN gateway VIPs, so it cannot reach the backend subnets — local health checks all time out. This is not an outage: the master node serves the traffic. After a failover (this node takes over the VIP), the backends will show UP here.",
     "maintenanceAlert": "{{count}} domain(s) in maintenance mode",
     "onboardingTitle": "Welcome to EdgeGuard",
     "onboardingIntro": "Fresh box — here are the next steps to route customer traffic:",