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.ExternalSystemGateway.Tests/DatabaseGatewayTests.cs

@@ -53,4 +53,26 @@ public class DatabaseGatewayTests
         await Assert.ThrowsAsync<InvalidOperationException>(
             () => gateway.CachedWriteAsync("nonexistent", "INSERT INTO t VALUES (1)"));
     }
+
+    // ── ExternalSystemGateway-001: buffered CachedDbWrite delivery handler ──
+
+    [Fact]
+    public async Task DeliverBuffered_ConnectionNoLongerExists_ReturnsFalseSoMessageParks()
+    {
+        _repository.GetAllDatabaseConnectionsAsync().Returns(new List<DatabaseConnectionDefinition>());
+        var gateway = new DatabaseGateway(_repository, NullLogger<DatabaseGateway>.Instance);
+
+        var message = new ScadaLink.StoreAndForward.StoreAndForwardMessage
+        {
+            Id = Guid.NewGuid().ToString("N"),
+            Category = ScadaLink.Commons.Types.Enums.StoreAndForwardCategory.CachedDbWrite,
+            Target = "gone-db",
+            PayloadJson =
+                """{"ConnectionName":"gone-db","Sql":"INSERT INTO t VALUES (1)","Parameters":null}""",
+        };
+
+        var delivered = await gateway.DeliverBufferedAsync(message);
+
+        Assert.False(delivered); // permanent — the S&F engine parks the message
+    }
 }