New per-admin-node actor that subscribes to the fleet-status DistributedPubSub
topic + forwards every FleetStatusChanged snapshot to all SignalR clients
connected to FleetStatusHub via IHubContext.
Wired via WithOtOpcUaSignalRBridges (new AkkaConfigurationBuilder extension in
AdminUI.Hubs) — Program.cs calls it inside the if(hasAdmin) block alongside
WithOtOpcUaControlPlaneSingletons.
Per-node subscription rather than cluster-singleton: every admin node forwards
its own snapshots to its own connected clients. Simpler than singleton
coordination + acceptable because the messages are small and SignalR fan-out
is per-node anyway.
