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/Actors/DebugStreamBridgeActor.cs

@@ -0,0 +1,100 @@
+using Akka.Actor;
+using Akka.Event;
+using ScadaLink.Commons.Messages.DebugView;
+using ScadaLink.Commons.Messages.Streaming;
+
+namespace ScadaLink.Communication.Actors;
+
+/// <summary>
+/// Persistent actor (one per active debug session) on the central side.
+/// Sends SubscribeDebugViewRequest to the site via CentralCommunicationActor (with THIS actor
+/// as the Sender), so the site's InstanceActor registers this actor as the debug subscriber.
+/// Stream events flow back via Akka remoting and are forwarded to the consumer via callbacks.
+/// </summary>
+public class DebugStreamBridgeActor : ReceiveActor
+{
+    private readonly ILoggingAdapter _log = Context.GetLogger();
+    private readonly string _siteIdentifier;
+    private readonly string _instanceUniqueName;
+    private readonly string _correlationId;
+    private readonly IActorRef _centralCommunicationActor;
+    private readonly Action<object> _onEvent;
+    private readonly Action _onTerminated;
+
+    public DebugStreamBridgeActor(
+        string siteIdentifier,
+        string instanceUniqueName,
+        string correlationId,
+        IActorRef centralCommunicationActor,
+        Action<object> onEvent,
+        Action onTerminated)
+    {
+        _siteIdentifier = siteIdentifier;
+        _instanceUniqueName = instanceUniqueName;
+        _correlationId = correlationId;
+        _centralCommunicationActor = centralCommunicationActor;
+        _onEvent = onEvent;
+        _onTerminated = onTerminated;
+
+        // Initial snapshot response from the site
+        Receive<DebugViewSnapshot>(snapshot =>
+        {
+            _log.Info("Received initial snapshot for {0} ({1} attrs, {2} alarms)",
+                _instanceUniqueName, snapshot.AttributeValues.Count, snapshot.AlarmStates.Count);
+            _onEvent(snapshot);
+        });
+
+        // Ongoing stream events from the site's InstanceActor
+        Receive<AttributeValueChanged>(changed => _onEvent(changed));
+        Receive<AlarmStateChanged>(changed => _onEvent(changed));
+
+        // Consumer requests stop
+        Receive<StopDebugStream>(_ =>
+        {
+            _log.Info("Stopping debug stream for {0}", _instanceUniqueName);
+            SendUnsubscribe();
+            Context.Stop(Self);
+        });
+
+        // Site disconnected — CentralCommunicationActor notifies us
+        Receive<DebugStreamTerminated>(msg =>
+        {
+            _log.Warning("Debug stream terminated for {0} (site {1} disconnected)", _instanceUniqueName, msg.SiteId);
+            _onTerminated();
+            Context.Stop(Self);
+        });
+
+        // Orphan safety net — if nobody stops us within 5 minutes, self-terminate
+        Context.SetReceiveTimeout(TimeSpan.FromMinutes(5));
+        Receive<ReceiveTimeout>(_ =>
+        {
+            _log.Warning("Debug stream for {0} timed out (orphaned session), stopping", _instanceUniqueName);
+            SendUnsubscribe();
+            _onTerminated();
+            Context.Stop(Self);
+        });
+    }
+
+    protected override void PreStart()
+    {
+        _log.Info("Starting debug stream bridge for {0} on site {1}", _instanceUniqueName, _siteIdentifier);
+
+        // Send subscribe request via CentralCommunicationActor.
+        // THIS actor is the Sender, so the site's InstanceActor registers us as the subscriber.
+        var request = new SubscribeDebugViewRequest(_instanceUniqueName, _correlationId);
+        var envelope = new SiteEnvelope(_siteIdentifier, request);
+        _centralCommunicationActor.Tell(envelope, Self);
+    }
+
+    private void SendUnsubscribe()
+    {
+        var request = new UnsubscribeDebugViewRequest(_instanceUniqueName, _correlationId);
+        var envelope = new SiteEnvelope(_siteIdentifier, request);
+        _centralCommunicationActor.Tell(envelope, Self);
+    }
+}
+
+/// <summary>
+/// Message sent to a DebugStreamBridgeActor to stop the debug stream session.
+/// </summary>
+public record StopDebugStream;