fix(store-and-forward): resolve S&F delivery + replication wiring (3 Critical findings)

Resolves StoreAndForward-001, ExternalSystemGateway-001, NotificationService-001
— one systemic gap where buffered messages were persisted but never delivered,
and the active node never replicated its buffer to the standby.

Delivery handlers (ExternalSystemGateway-001 / NotificationService-001):
- AkkaHostedService registers delivery handlers for the ExternalSystem,
  CachedDbWrite and Notification categories after StoreAndForwardService starts;
  each resolves its scoped consumer in a fresh DI scope.
- ExternalSystemClient, DatabaseGateway and NotificationDeliveryService each
  gain a DeliverBufferedAsync method: re-resolve the target and re-attempt
  delivery, returning true/false/throwing per the transient-vs-permanent contract.
- EnqueueAsync gains an attemptImmediateDelivery flag; CachedCallAsync and
  NotificationDeliveryService.SendAsync pass false (they already attempted
  delivery themselves) so registering a handler does not dispatch twice.

Replication (StoreAndForward-001):
- ReplicationService is injected into StoreAndForwardService; a new BufferAsync
  helper replicates every enqueue, and successful-retry removes and parks are
  replicated too. Fire-and-forget, no-op when replication is disabled.

Tests: StoreAndForwardReplicationTests (Add/Remove/Park observed),
attemptImmediateDelivery behaviour, and DeliverBufferedAsync paths for each
consumer. Full solution builds; StoreAndForward/ExternalSystemGateway/
NotificationService suites green.

tests/ScadaLink.NotificationService.Tests/NotificationDeliveryServiceTests.cs

@@ -192,4 +192,37 @@ public class NotificationDeliveryServiceTests
         Assert.True(result.Success);
         Assert.True(result.WasBuffered);
     }
+
+    // ── NotificationService-001: buffered-notification delivery handler ──
+
+    private static StoreAndForward.StoreAndForwardMessage BufferedNotification(string listName) =>
+        new()
+        {
+            Id = Guid.NewGuid().ToString("N"),
+            Category = ScadaLink.Commons.Types.Enums.StoreAndForwardCategory.Notification,
+            Target = listName,
+            PayloadJson = $$"""{"ListName":"{{listName}}","Subject":"Alert","Message":"Body"}""",
+        };
+
+    [Fact]
+    public async Task DeliverBuffered_HappyPath_ReturnsTrue()
+    {
+        SetupHappyPath();
+        var service = CreateService();
+
+        var delivered = await service.DeliverBufferedAsync(BufferedNotification("ops-team"));
+
+        Assert.True(delivered);
+    }
+
+    [Fact]
+    public async Task DeliverBuffered_ListNoLongerExists_ReturnsFalseSoMessageParks()
+    {
+        _repository.GetListByNameAsync("gone-list").Returns((NotificationList?)null);
+        var service = CreateService();
+
+        var delivered = await service.DeliverBufferedAsync(BufferedNotification("gone-list"));
+
+        Assert.False(delivered); // permanent — the S&F engine parks the message
+    }
 }