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
+    }
 }

tests/ScadaLink.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs

@@ -153,6 +153,69 @@ public class ExternalSystemClientTests
         Assert.False(result.WasBuffered);
     }
 
+    // ── ExternalSystemGateway-001: buffered-call delivery handler ──
+
+    private static ScadaLink.StoreAndForward.StoreAndForwardMessage BufferedCall(
+        string systemName, string methodName) =>
+        new()
+        {
+            Id = Guid.NewGuid().ToString("N"),
+            Category = ScadaLink.Commons.Types.Enums.StoreAndForwardCategory.ExternalSystem,
+            Target = systemName,
+            PayloadJson =
+                $$"""{"SystemName":"{{systemName}}","MethodName":"{{methodName}}","Parameters":null}""",
+        };
+
+    [Fact]
+    public async Task DeliverBuffered_SuccessfulHttp_ReturnsTrue()
+    {
+        var system = new ExternalSystemDefinition("TestAPI", "https://api.example.com", "none") { Id = 1 };
+        var method = new ExternalSystemMethod("getData", "GET", "/data") { Id = 1, ExternalSystemDefinitionId = 1 };
+        _repository.GetAllExternalSystemsAsync().Returns(new List<ExternalSystemDefinition> { system });
+        _repository.GetMethodsByExternalSystemIdAsync(1).Returns(new List<ExternalSystemMethod> { method });
+
+        var httpClient = new HttpClient(new MockHttpMessageHandler(HttpStatusCode.OK, "{\"ok\":true}"));
+        _httpClientFactory.CreateClient(Arg.Any<string>()).Returns(httpClient);
+
+        var client = new ExternalSystemClient(
+            _httpClientFactory, _repository, NullLogger<ExternalSystemClient>.Instance);
+
+        var delivered = await client.DeliverBufferedAsync(BufferedCall("TestAPI", "getData"));
+
+        Assert.True(delivered);
+    }
+
+    [Fact]
+    public async Task DeliverBuffered_SystemNoLongerExists_ReturnsFalseSoMessageParks()
+    {
+        _repository.GetAllExternalSystemsAsync().Returns(new List<ExternalSystemDefinition>());
+
+        var client = new ExternalSystemClient(
+            _httpClientFactory, _repository, NullLogger<ExternalSystemClient>.Instance);
+
+        var delivered = await client.DeliverBufferedAsync(BufferedCall("GoneAPI", "method"));
+
+        Assert.False(delivered); // permanent — the S&F engine parks the message
+    }
+
+    [Fact]
+    public async Task DeliverBuffered_Transient500_ThrowsSoEngineRetries()
+    {
+        var system = new ExternalSystemDefinition("TestAPI", "https://api.example.com", "none") { Id = 1 };
+        var method = new ExternalSystemMethod("failMethod", "POST", "/fail") { Id = 1, ExternalSystemDefinitionId = 1 };
+        _repository.GetAllExternalSystemsAsync().Returns(new List<ExternalSystemDefinition> { system });
+        _repository.GetMethodsByExternalSystemIdAsync(1).Returns(new List<ExternalSystemMethod> { method });
+
+        var httpClient = new HttpClient(new MockHttpMessageHandler(HttpStatusCode.InternalServerError, "boom"));
+        _httpClientFactory.CreateClient(Arg.Any<string>()).Returns(httpClient);
+
+        var client = new ExternalSystemClient(
+            _httpClientFactory, _repository, NullLogger<ExternalSystemClient>.Instance);
+
+        await Assert.ThrowsAsync<TransientExternalSystemException>(
+            () => client.DeliverBufferedAsync(BufferedCall("TestAPI", "failMethod")));
+    }
+
     /// <summary>
     /// Test helper: mock HTTP message handler.
     /// </summary>