feat: add JoeAppEngine OPC UA nodes, fix DCL auto-reconnect and quality push

- Add JoeAppEngine folder to OPC UA nodes.json (BTCS, AlarmCntsBySeverity, Scheduler/ScanTime)
- Fix DataConnectionActor: capture Self in PreStart for use from non-actor threads,
  preventing Self.Tell failure in Disconnected event handler
- Implement InstanceActor.HandleConnectionQualityChanged to mark attributes Bad on disconnect
- Fix LmxFakeProxy TagMapper to serialize arrays as JSON instead of "System.Int32[]"
- Allow DataType and DataSourceReference updates in TemplateService.UpdateAttributeAsync
- Update test_infra_opcua.md with JoeAppEngine documentation

src/ScadaLink.Host/Health/AkkaClusterHealthCheck.cs

@@ -1,19 +1,42 @@
+using Akka.Actor;
+using Akka.Cluster;
 using Microsoft.Extensions.Diagnostics.HealthChecks;
 
 namespace ScadaLink.Host.Health;
 
 /// <summary>
-/// Health check that verifies Akka.NET cluster membership.
-/// Initially returns healthy; will be refined when Akka cluster integration is complete.
+/// Health check that verifies this node is an active member of the Akka.NET cluster.
+/// Returns healthy only if the node's self-member status is Up or Joining.
 /// </summary>
 public class AkkaClusterHealthCheck : IHealthCheck
 {
+    private readonly ActorSystem? _system;
+
+    public AkkaClusterHealthCheck(ActorSystem? system = null)
+    {
+        _system = system;
+    }
+
     public Task<HealthCheckResult> CheckHealthAsync(
         HealthCheckContext context,
         CancellationToken cancellationToken = default)
     {
-        // TODO: Query Akka Cluster.Get(system).State to verify this node is Up.
-        // For now, return healthy as Akka cluster wiring is being established.
-        return Task.FromResult(HealthCheckResult.Healthy("Akka cluster health check placeholder."));
+        if (_system == null)
+            return Task.FromResult(HealthCheckResult.Degraded("ActorSystem not yet available."));
+
+        var cluster = Cluster.Get(_system);
+        var status = cluster.SelfMember.Status;
+
+        var result = status switch
+        {
+            MemberStatus.Up or MemberStatus.Joining =>
+                HealthCheckResult.Healthy($"Akka cluster member status: {status}"),
+            MemberStatus.Leaving or MemberStatus.Exiting =>
+                HealthCheckResult.Degraded($"Akka cluster member status: {status}"),
+            _ =>
+                HealthCheckResult.Unhealthy($"Akka cluster member status: {status}")
+        };
+
+        return Task.FromResult(result);
     }
 }