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.StoreAndForward.Tests/NotificationForwarderTests.cs

@@ -18,19 +18,26 @@ public class NotificationForwarderTests : TestKit
 
     /// <summary>
     /// Builds a buffered notification S&amp;F message whose payload matches the shape
-    /// produced by the site NotificationDeliveryService enqueue path.
+    /// produced by the site <c>Notify.Send</c> enqueue path (Task 19): a serialized
+    /// <see cref="NotificationSubmit"/> carrying a script-generated
+    /// <see cref="NotificationSubmit.NotificationId"/>. The S&amp;F message
+    /// <see cref="StoreAndForwardMessage.Id"/> equals that same id.
     /// </summary>
     private static StoreAndForwardMessage BufferedNotification(
         string id = "msg-1", string listName = "Operators",
         string subject = "Pump alarm", string message = "Pump 3 tripped",
-        string? originInstance = "Plant.Pump3")
+        string? originInstance = "Plant.Pump3", string? sourceScript = "alarmScript")
     {
-        var payload = JsonSerializer.Serialize(new
-        {
-            ListName = listName,
-            Subject = subject,
-            Message = message
-        });
+        var payload = JsonSerializer.Serialize(new NotificationSubmit(
+            NotificationId: id,
+            ListName: listName,
+            Subject: subject,
+            Body: message,
+            // SourceSiteId is re-stamped by the forwarder; the enqueue side leaves it blank.
+            SourceSiteId: string.Empty,
+            SourceInstanceId: originInstance,
+            SourceScript: sourceScript,
+            SiteEnqueuedAt: DateTimeOffset.UtcNow));
         return new StoreAndForwardMessage
         {
             Id = id,
@@ -57,11 +64,15 @@ public class NotificationForwarderTests : TestKit
         // The central target receives a NotificationSubmit whose fields map from the
         // buffered payload; reply Accepted so the handler completes as delivered.
         var submit = centralProbe.ExpectMsg<NotificationSubmit>();
+        Assert.Equal("msg-1", submit.NotificationId);
         Assert.Equal("Operators", submit.ListName);
         Assert.Equal("Pump alarm", submit.Subject);
         Assert.Equal("Pump 3 tripped", submit.Body);
+        // SourceSiteId is re-stamped by the forwarder from its own site id.
         Assert.Equal("site-7", submit.SourceSiteId);
         Assert.Equal("Plant.Pump3", submit.SourceInstanceId);
+        // The originating script travels through from the buffered payload.
+        Assert.Equal("alarmScript", submit.SourceScript);
         centralProbe.Reply(new NotificationSubmitAck(submit.NotificationId, Accepted: true, Error: null));
 
         Assert.True(await deliverTask);
@@ -76,12 +87,15 @@ public class NotificationForwarderTests : TestKit
 
         // A buffered payload carrying an empty-string ListName: the empty value must not
         // be forwarded — the forwarder falls back to the S&F message Target instead.
-        var payload = JsonSerializer.Serialize(new
-        {
-            ListName = "",
-            Subject = "Pump alarm",
-            Message = "Pump 3 tripped"
-        });
+        var payload = JsonSerializer.Serialize(new NotificationSubmit(
+            NotificationId: "msg-empty-list",
+            ListName: "",
+            Subject: "Pump alarm",
+            Body: "Pump 3 tripped",
+            SourceSiteId: string.Empty,
+            SourceInstanceId: "Plant.Pump3",
+            SourceScript: null,
+            SiteEnqueuedAt: DateTimeOffset.UtcNow));
         var msg = new StoreAndForwardMessage
         {
             Id = "msg-empty-list",