feat(dcl): add active endpoint to health reports and log failover events

Add ActiveEndpoint field to DataConnectionHealthReport showing which
endpoint is active (Primary, Backup, or Primary with no backup configured).
Log failover transitions and connection restoration events to the site
event log via ISiteEventLogger, passed as an optional parameter through
the actor hierarchy for backwards compatibility.

src/ScadaLink.DataConnectionLayer/Actors/DataConnectionManagerActor.cs

@@ -3,6 +3,7 @@ using Akka.Event;
 using ScadaLink.Commons.Interfaces.Protocol;
 using ScadaLink.Commons.Messages.DataConnection;
 using ScadaLink.HealthMonitoring;
+using ScadaLink.SiteEventLogging;
 
 namespace ScadaLink.DataConnectionLayer.Actors;
 
@@ -17,16 +18,19 @@ public class DataConnectionManagerActor : ReceiveActor
     private readonly IDataConnectionFactory _factory;
     private readonly DataConnectionOptions _options;
     private readonly ISiteHealthCollector _healthCollector;
+    private readonly ISiteEventLogger? _siteEventLogger;
     private readonly Dictionary<string, IActorRef> _connectionActors = new();
 
     public DataConnectionManagerActor(
         IDataConnectionFactory factory,
         DataConnectionOptions options,
-        ISiteHealthCollector healthCollector)
+        ISiteHealthCollector healthCollector,
+        ISiteEventLogger? siteEventLogger = null)
     {
         _factory = factory;
         _options = options;
         _healthCollector = healthCollector;
+        _siteEventLogger = siteEventLogger;
 
         Receive<CreateConnectionCommand>(HandleCreateConnection);
         Receive<SubscribeTagsRequest>(HandleRoute);
@@ -52,7 +56,8 @@ public class DataConnectionManagerActor : ReceiveActor
             _factory, command.ProtocolType,
             command.PrimaryConnectionDetails,
             command.BackupConnectionDetails,
-            command.FailoverRetryCount));
+            command.FailoverRetryCount,
+            _siteEventLogger));
 
         // Sanitize name for Akka actor path (replace spaces and invalid chars)
         var actorName = new string(command.ConnectionName