Instrument the historian plugin with runtime query health counters and read-only cluster failover so operators can detect silent query degradation and keep serving history when a single cluster node goes down

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.LmxOpcUa.Host/appsettings.json

@@ -75,6 +75,8 @@
   "Historian": {
     "Enabled": false,
     "ServerName": "localhost",
+    "ServerNames": [],
+    "FailureCooldownSeconds": 60,
     "IntegratedSecurity": true,
     "UserName": null,
     "Password": null,