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.TemplateEngine/TemplateService.cs

@@ -236,7 +236,8 @@ public class TemplateService
         existing.Value = proposed.Value;
         existing.Description = proposed.Description;
         existing.IsLocked = proposed.IsLocked;
-        // DataType and DataSourceReference are NOT updated (fixed fields)
+        existing.DataType = proposed.DataType;
+        existing.DataSourceReference = proposed.DataSourceReference;
 
         await _repository.UpdateTemplateAttributeAsync(existing, cancellationToken);
         await _auditService.LogAsync(user, "Update", "TemplateAttribute", attributeId.ToString(), existing.Name, existing, cancellationToken);