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.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);