chore(deploy): CentralFetchBaseUrl appsettings + RUNBOOK

Populates ScadaBridge:Communication:CentralFetchBaseUrl in every central
appsettings for the notify-and-fetch deploy flow.  An empty value now
causes a fail-fast on deploy; this config prevents that regression.

- docker/: http://scadabridge-traefik  (in-network Traefik LB, port 80)
- docker-env2/: http://scadabridge-env2-traefik  (env2 Traefik LB, port 80)
- src/Host base: http://localhost:5000  (ASP.NET Core default for single-host dev)
- deploy/wonder-app-vd03: http://localhost:8085  (gitignored; edited in main repo)
  RUNBOOK Upgrading section updated with note on this setting.

docker/central-node-a/appsettings.Central.json

@@ -45,7 +45,8 @@
       "LifecycleTimeout": "00:00:30",
       "QueryTimeout": "00:00:30",
       "TransportHeartbeatInterval": "00:00:05",
-      "TransportFailureThreshold": "00:00:15"
+      "TransportFailureThreshold": "00:00:15",
+      "CentralFetchBaseUrl": "http://scadabridge-traefik"
     },
     "HealthMonitoring": {
       "ReportInterval": "00:00:30",