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