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.CentralUI/Components/Pages/Monitoring/Health.razor

@@ -90,11 +90,25 @@
                                 <h6 class="text-muted mb-2 border-bottom pb-1">Nodes</h6>
                                 <table class="table table-sm table-borderless mb-0">
                                     <tbody>
+                                        @if (report.ClusterNodes is { Count: > 0 })
+                                        {
+                                            @foreach (var node in report.ClusterNodes)
+                                            {
+                                                <tr>
+                                                    <td class="small">@node.Hostname</td>
+                                                    <td><span class="badge @(node.IsOnline ? "bg-success" : "bg-danger")">@(node.IsOnline ? "Online" : "Offline")</span></td>
+                                                    <td><span class="badge @(node.Role == "Primary" ? "bg-primary" : "bg-secondary")">@node.Role</span></td>
+                                                </tr>
+                                            }
+                                        }
+                                        else
+                                        {
                                             <tr>
                                                 <td class="small">@(report.NodeHostname != "" ? report.NodeHostname : "Node")</td>
                                                 <td><span class="badge @(state.IsOnline ? "bg-success" : "bg-danger")">@(state.IsOnline ? "Online" : "Offline")</span></td>
                                                 <td><span class="badge @(report.NodeRole == "Active" ? "bg-primary" : "bg-secondary")">@(report.NodeRole == "Active" ? "Primary" : "Standby")</span></td>
                                             </tr>
+                                        }
                                     </tbody>
                                 </table>
                             </div>

src/ScadaLink.Commons/Messages/Health/NodeStatus.cs

@@ -0,0 +1,3 @@
+namespace ScadaLink.Commons.Messages.Health;
+
+public record NodeStatus(string Hostname, bool IsOnline, string Role);

src/ScadaLink.Commons/Messages/Health/SiteHealthReport.cs

@@ -19,4 +19,5 @@ public record SiteHealthReport(
     string NodeHostname = "",
     IReadOnlyDictionary<string, string>? DataConnectionEndpoints = null,
     IReadOnlyDictionary<string, TagQualityCounts>? DataConnectionTagQuality = null,
-    int ParkedMessageCount = 0);
+    int ParkedMessageCount = 0,
+    IReadOnlyList<NodeStatus>? ClusterNodes = null);

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());
     }
 }

src/ScadaLink.Host/Health/AkkaClusterNodeProvider.cs

@@ -0,0 +1,62 @@
+using Akka.Actor;
+using Akka.Cluster;
+using ScadaLink.Commons.Messages.Health;
+using ScadaLink.HealthMonitoring;
+using ScadaLink.Host.Actors;
+
+namespace ScadaLink.Host.Health;
+
+/// <summary>
+/// Provides cluster node statuses from Akka.NET cluster membership for health reporting.
+/// </summary>
+public class AkkaClusterNodeProvider : IClusterNodeProvider
+{
+    private readonly AkkaHostedService _akkaService;
+    private readonly string _siteRole;
+
+    public AkkaClusterNodeProvider(AkkaHostedService akkaService, string siteRole)
+    {
+        _akkaService = akkaService;
+        _siteRole = siteRole;
+    }
+
+    public IReadOnlyList<NodeStatus> GetClusterNodes()
+    {
+        var system = _akkaService.ActorSystem;
+        if (system == null) return [];
+
+        var cluster = Cluster.Get(system);
+        var selfAddress = cluster.SelfAddress;
+        var leader = cluster.State.Leader;
+
+        var nodes = new List<NodeStatus>();
+        foreach (var member in cluster.State.Members)
+        {
+            if (!member.HasRole(_siteRole))
+                continue;
+
+            var hostname = member.Address.Host ?? member.Address.ToString();
+            var isOnline = member.Status == MemberStatus.Up;
+            var isLeader = member.Address.Equals(leader);
+            var role = isLeader ? "Primary" : "Standby";
+
+            nodes.Add(new NodeStatus(hostname, isOnline, role));
+        }
+
+        // If we have unreachable members, add them as offline
+        foreach (var unreachable in cluster.State.Unreachable)
+        {
+            if (!unreachable.HasRole(_siteRole))
+                continue;
+
+            // Don't duplicate if already in members list
+            if (nodes.Any(n => n.Hostname == (unreachable.Address.Host ?? unreachable.Address.ToString())))
+                continue;
+
+            var hostname = unreachable.Address.Host ?? unreachable.Address.ToString();
+            nodes.Add(new NodeStatus(hostname, false, "Standby"));
+        }
+
+        return nodes;
+    }
+}

src/ScadaLink.Host/SiteServiceRegistration.cs

@@ -4,6 +4,7 @@ using ScadaLink.DataConnectionLayer;
 using ScadaLink.ExternalSystemGateway;
 using ScadaLink.HealthMonitoring;
 using ScadaLink.Host.Actors;
+using ScadaLink.Host.Health;
 using ScadaLink.NotificationService;
 using ScadaLink.SiteEventLogging;
 using ScadaLink.SiteRuntime;
@@ -42,6 +43,15 @@ public static class SiteServiceRegistration
         services.AddSingleton<AkkaHostedService>();
         services.AddHostedService(sp => sp.GetRequiredService<AkkaHostedService>());
 
+        // Cluster node status provider for health reports
+        services.AddSingleton<IClusterNodeProvider>(sp =>
+        {
+            var akkaService = sp.GetRequiredService<AkkaHostedService>();
+            var nodeOptions = sp.GetRequiredService<Microsoft.Extensions.Options.IOptions<NodeOptions>>().Value;
+            var siteRole = $"site-{nodeOptions.SiteId}";
+            return new AkkaClusterNodeProvider(akkaService, siteRole);
+        });
+
         // Options binding
         BindSharedOptions(services, config);
         services.Configure<SiteRuntimeOptions>(config.GetSection("ScadaLink:SiteRuntime"));