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/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();
+}