feat(notification-outbox): async Notify.Send with status handle

Notify.To(list).Send(subject,body) now generates a NotificationId GUID,
enqueues a Notification-category message into the site Store-and-Forward
Engine, and returns the NotificationId immediately (Task<string>). The
NotificationId is the single idempotency key end-to-end: it is the S&F
message Id, it is carried inside the buffered NotificationSubmit payload,
and it is the id the forwarder submits to central.

NotificationForwarder now deserializes the buffered payload as a
NotificationSubmit and reads NotificationId from it (re-stamping only the
site-owned SourceSiteId / SourceInstanceId), instead of deriving the id
from StoreAndForwardMessage.Id.

Adds NotifyHelper.Status(id): queries central via the site communication
actor; reports the site-local Forwarding state while the notification is
still buffered at the site, maps central's response when found, and
Unknown otherwise. Adds a NotificationDeliveryStatus record.

SiteCommunicationActor gains a NotificationStatusQuery forwarding handler
mirroring NotificationSubmit. StoreAndForwardService.EnqueueAsync gains an
optional messageId parameter and exposes GetMessageByIdAsync.

tests/ScadaLink.Communication.Tests/SiteCommunicationActorTests.cs

@@ -155,6 +155,51 @@ public class SiteCommunicationActorTests : TestKit
         ExpectMsg<NotificationSubmitAck>(ack => ack.NotificationId == "notif-2" && !ack.Accepted);
     }
 
+    [Fact]
+    public void NotificationStatusQuery_WithCentralClient_ForwardedToCentralAndResponseRoutedBack()
+    {
+        // Notify.Status(id) issues a NotificationStatusQuery; the site actor forwards it
+        // to central over the ClusterClient command/control transport and the central
+        // response must route back to the original sender (the helper's Ask).
+        var dmProbe = CreateTestProbe();
+        var centralClientProbe = CreateTestProbe();
+        var siteActor = Sys.ActorOf(Props.Create(() =>
+            new SiteCommunicationActor("site1", _options, dmProbe.Ref)));
+
+        siteActor.Tell(new RegisterCentralClient(centralClientProbe.Ref));
+
+        var query = new NotificationStatusQuery("corr-99", "notif-1");
+        siteActor.Tell(query);
+
+        var send = centralClientProbe.FishForMessage<ClusterClient.Send>(
+            s => s.Message is NotificationStatusQuery);
+        Assert.Equal("/user/central-communication", send.Path);
+        var forwarded = Assert.IsType<NotificationStatusQuery>(send.Message);
+        Assert.Equal("notif-1", forwarded.NotificationId);
+
+        // The response is sent to the ClusterClient.Send's Sender — replying as that
+        // probe must land back at the test actor (the original Tell sender).
+        centralClientProbe.Reply(new NotificationStatusResponse(
+            "corr-99", Found: true, Status: "Delivered", RetryCount: 0,
+            LastError: null, DeliveredAt: DateTimeOffset.UtcNow));
+        ExpectMsg<NotificationStatusResponse>(r => r.CorrelationId == "corr-99" && r.Found);
+    }
+
+    [Fact]
+    public void NotificationStatusQuery_WithoutCentralClient_RepliesWithNotFound()
+    {
+        // No ClusterClient registered yet: the query cannot reach central, so the actor
+        // replies Found: false. Notify.Status then falls back to the site S&F buffer.
+        var dmProbe = CreateTestProbe();
+        var siteActor = Sys.ActorOf(Props.Create(() =>
+            new SiteCommunicationActor("site1", _options, dmProbe.Ref)));
+
+        siteActor.Tell(new NotificationStatusQuery("corr-100", "notif-2"));
+
+        ExpectMsg<NotificationStatusResponse>(
+            r => r.CorrelationId == "corr-100" && !r.Found);
+    }
+
     [Fact]
     public void EventLogQuery_WithoutHandler_ReturnsFailure()
     {