feat: replace debug view polling with real-time SignalR streaming

The debug view polled every 2s by re-subscribing for full snapshots. Now a
persistent DebugStreamBridgeActor on central subscribes once and receives
incremental Akka stream events from the site, forwarding them to the Blazor
component via callbacks and to the CLI via a new SignalR hub at
/hubs/debug-stream. Adds `debug stream` CLI command with auto-reconnect.

src/ScadaLink.Communication/CommunicationService.cs

@@ -48,12 +48,17 @@ public class CommunicationService
         GetActor().Tell(new RefreshSiteAddresses());
     }
 
-    private IActorRef GetActor()
+    /// <summary>
+    /// Gets the central communication actor reference. Throws if not yet initialized.
+    /// </summary>
+    public IActorRef GetCommunicationActor()
     {
         return _centralCommunicationActor
             ?? throw new InvalidOperationException("CommunicationService not initialized. CentralCommunicationActor not set.");
     }
 
+    private IActorRef GetActor() => GetCommunicationActor();
+
     // ── Pattern 1: Instance Deployment ──
 
     public async Task<DeploymentStatusResponse> DeployInstanceAsync(