Phase 1 WP-11–22: Host infrastructure, Blazor Server UI, and integration tests

Host infrastructure (WP-11–17):
- StartupValidator with 19 validation rules
- /health/ready endpoint with DB + Akka health checks
- Akka.NET bootstrap via AkkaHostedService (HOCON config, cluster, remoting, SBR)
- Serilog with SiteId/NodeHostname/NodeRole enrichment
- DeadLetterMonitorActor with count tracking
- CoordinatedShutdown wiring (no Environment.Exit)
- Windows Service support (UseWindowsService)

Central UI (WP-18–21):
- Blazor Server shell with Bootstrap 5, role-aware NavMenu
- Login/logout flow (LDAP auth → JWT → HTTP-only cookie)
- CookieAuthenticationStateProvider with idle timeout
- LDAP group mapping CRUD page (Admin role)
- Route guards with Authorize attributes per role
- SignalR reconnection overlay for failover

Integration tests (WP-22):
- Startup validation, auth flow, audit transactions, readiness gating
186 tests pass (1 skipped: LDAP integration), zero warnings.

tests/ScadaLink.Host.Tests/DeadLetterMonitorTests.cs

@@ -0,0 +1,74 @@
+using Akka.Actor;
+using Akka.Event;
+using Akka.TestKit.Xunit2;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using ScadaLink.Host.Actors;
+
+namespace ScadaLink.Host.Tests;
+
+/// <summary>
+/// WP-15: Tests for DeadLetterMonitorActor.
+/// </summary>
+public class DeadLetterMonitorTests : TestKit
+{
+    private readonly ILogger<DeadLetterMonitorActor> _logger =
+        NullLoggerFactory.Instance.CreateLogger<DeadLetterMonitorActor>();
+
+    [Fact]
+    public void DeadLetterMonitor_StartsWithZeroCount()
+    {
+        var monitor = Sys.ActorOf(Props.Create(() => new DeadLetterMonitorActor(_logger)));
+
+        monitor.Tell(GetDeadLetterCount.Instance);
+        var response = ExpectMsg<DeadLetterCountResponse>();
+
+        Assert.Equal(0, response.Count);
+    }
+
+    [Fact]
+    public void DeadLetterMonitor_IncrementsOnDeadLetter()
+    {
+        var monitor = Sys.ActorOf(Props.Create(() => new DeadLetterMonitorActor(_logger)));
+
+        // Ensure actor has started and subscribed by sending a message and waiting for response
+        monitor.Tell(GetDeadLetterCount.Instance);
+        ExpectMsg<DeadLetterCountResponse>();
+
+        // Now publish dead letters — actor is guaranteed to be subscribed
+        Sys.EventStream.Publish(new DeadLetter("test-message-1", Sys.DeadLetters, Sys.DeadLetters));
+        Sys.EventStream.Publish(new DeadLetter("test-message-2", Sys.DeadLetters, Sys.DeadLetters));
+
+        // Use AwaitAssert to handle async event delivery
+        AwaitAssert(() =>
+        {
+            monitor.Tell(GetDeadLetterCount.Instance);
+            var response = ExpectMsg<DeadLetterCountResponse>();
+            Assert.Equal(2, response.Count);
+        });
+    }
+
+    [Fact]
+    public void DeadLetterMonitor_CountAccumulates()
+    {
+        var monitor = Sys.ActorOf(Props.Create(() => new DeadLetterMonitorActor(_logger)));
+
+        // Ensure actor is started and subscribed
+        monitor.Tell(GetDeadLetterCount.Instance);
+        ExpectMsg<DeadLetterCountResponse>();
+
+        // Send 5 dead letters
+        for (var i = 0; i < 5; i++)
+        {
+            Sys.EventStream.Publish(
+                new DeadLetter($"message-{i}", Sys.DeadLetters, Sys.DeadLetters));
+        }
+
+        AwaitAssert(() =>
+        {
+            monitor.Tell(GetDeadLetterCount.Instance);
+            var response = ExpectMsg<DeadLetterCountResponse>();
+            Assert.Equal(5, response.Count);
+        });
+    }
+}