feat(health): show all cluster nodes (online/offline, primary/standby) in health dashboard

Add NodeStatus record, IClusterNodeProvider interface, and AkkaClusterNodeProvider
that queries Akka cluster membership for all site-role nodes. HealthReportSender
populates ClusterNodes before each report. UI shows a row per node with
hostname, Online/Offline badge, and Primary/Standby badge. Falls back to
single-node display if ClusterNodes is not populated.

src/ScadaLink.HealthMonitoring/HealthReportSender.cs

@@ -18,6 +18,7 @@ public class HealthReportSender : BackgroundService
     private readonly ILogger<HealthReportSender> _logger;
     private readonly string _siteId;
     private readonly StoreAndForwardStorage? _sfStorage;
+    private readonly IClusterNodeProvider? _clusterNodeProvider;
     private long _sequenceNumber;
 
     public HealthReportSender(
@@ -26,7 +27,8 @@ public class HealthReportSender : BackgroundService
         IOptions<HealthMonitoringOptions> options,
         ILogger<HealthReportSender> logger,
         ISiteIdentityProvider siteIdentityProvider,
-        StoreAndForwardStorage? sfStorage = null)
+        StoreAndForwardStorage? sfStorage = null,
+        IClusterNodeProvider? clusterNodeProvider = null)
     {
         _collector = collector;
         _transport = transport;
@@ -34,6 +36,7 @@ public class HealthReportSender : BackgroundService
         _logger = logger;
         _siteId = siteIdentityProvider.SiteId;
         _sfStorage = sfStorage;
+        _clusterNodeProvider = clusterNodeProvider;
     }
 
     /// <summary>
@@ -58,6 +61,15 @@ public class HealthReportSender : BackgroundService
                 if (!_collector.IsActiveNode)
                     continue;
 
+                if (_clusterNodeProvider != null)
+                {
+                    try
+                    {
+                        _collector.SetClusterNodes(_clusterNodeProvider.GetClusterNodes());
+                    }
+                    catch { /* Non-fatal */ }
+                }
+
                 if (_sfStorage != null)
                 {
                     try

src/ScadaLink.HealthMonitoring/IClusterNodeProvider.cs

@@ -0,0 +1,12 @@
+using ScadaLink.Commons.Messages.Health;
+
+namespace ScadaLink.HealthMonitoring;
+
+/// <summary>
+/// Provides cluster node status information for health reporting.
+/// Implemented by the Host project which has access to the Akka.NET actor system.
+/// </summary>
+public interface IClusterNodeProvider
+{
+    IReadOnlyList<NodeStatus> GetClusterNodes();
+}

src/ScadaLink.HealthMonitoring/ISiteHealthCollector.cs

@@ -21,6 +21,7 @@ public interface ISiteHealthCollector
     void SetInstanceCounts(int deployed, int enabled, int disabled);
     void SetParkedMessageCount(int count);
     void SetNodeHostname(string hostname);
+    void SetClusterNodes(IReadOnlyList<Commons.Messages.Health.NodeStatus> nodes);
     void SetActiveNode(bool isActive);
     bool IsActiveNode { get; }
     SiteHealthReport CollectReport(string siteId);

src/ScadaLink.HealthMonitoring/SiteHealthCollector.cs

@@ -21,6 +21,7 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _deployedInstanceCount, _enabledInstanceCount, _disabledInstanceCount;
     private int _parkedMessageCount;
     private volatile string _nodeHostname = "";
+    private volatile IReadOnlyList<Commons.Messages.Health.NodeStatus>? _clusterNodes;
     private volatile bool _isActiveNode;
 
     /// <summary>
@@ -94,6 +95,8 @@ public class SiteHealthCollector : ISiteHealthCollector
 
     public void SetNodeHostname(string hostname) => _nodeHostname = hostname;
 
+    public void SetClusterNodes(IReadOnlyList<Commons.Messages.Health.NodeStatus> nodes) => _clusterNodes = nodes;
+
     /// <summary>
     /// Set the current store-and-forward buffer depths snapshot.
     /// Called before report collection with data from the S&amp;F service.
@@ -159,6 +162,7 @@ public class SiteHealthCollector : ISiteHealthCollector
             NodeHostname: _nodeHostname,
             DataConnectionEndpoints: connectionEndpoints,
             DataConnectionTagQuality: tagQuality,
-            ParkedMessageCount: Interlocked.CompareExchange(ref _parkedMessageCount, 0, 0));
+            ParkedMessageCount: Interlocked.CompareExchange(ref _parkedMessageCount, 0, 0),
+            ClusterNodes: _clusterNodes?.ToList());
     }
 }