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.

src/ScadaLink.Communication/Actors/SiteCommunicationActor.cs

@@ -190,6 +190,30 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
                 new ClusterClient.Send("/user/central-communication", msg), Sender);
         });
 
+        // Notification Outbox: forward a Notify.Status query to the central cluster.
+        // The original Sender (the Notify helper's Ask) is forwarded as the
+        // ClusterClient.Send sender so the NotificationStatusResponse routes straight
+        // back to the waiting Ask, not here.
+        Receive<NotificationStatusQuery>(msg =>
+        {
+            if (_centralClient == null)
+            {
+                // No ClusterClient registered yet. Reply Found: false so Notify.Status
+                // falls back to the site S&F buffer to decide Forwarding vs Unknown.
+                _log.Warning(
+                    "Cannot forward NotificationStatusQuery {0} — no central ClusterClient registered",
+                    msg.NotificationId);
+                Sender.Tell(new NotificationStatusResponse(
+                    msg.CorrelationId, Found: false, Status: "Unknown",
+                    RetryCount: 0, LastError: null, DeliveredAt: null));
+                return;
+            }
+
+            _log.Debug("Forwarding NotificationStatusQuery {0} to central", msg.NotificationId);
+            _centralClient.Tell(
+                new ClusterClient.Send("/user/central-communication", msg), Sender);
+        });
+
         // Internal: send heartbeat tick
         Receive<SendHeartbeat>(_ => SendHeartbeatToCentral());