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.

src/ScadaLink.Host/Actors/DeadLetterMonitorActor.cs

@@ -0,0 +1,53 @@
+using Akka.Actor;
+using Akka.Event;
+using Microsoft.Extensions.Logging;
+
+namespace ScadaLink.Host.Actors;
+
+/// <summary>
+/// Subscribes to Akka.NET dead letter events, logs them, and tracks count
+/// for health monitoring integration.
+/// </summary>
+public class DeadLetterMonitorActor : ReceiveActor
+{
+    private long _deadLetterCount;
+
+    public DeadLetterMonitorActor(ILogger<DeadLetterMonitorActor> logger)
+    {
+        Receive<DeadLetter>(dl =>
+        {
+            _deadLetterCount++;
+            logger.LogWarning(
+                "Dead letter: {MessageType} from {Sender} to {Recipient}",
+                dl.Message.GetType().Name,
+                dl.Sender,
+                dl.Recipient);
+        });
+
+        Receive<GetDeadLetterCount>(_ => Sender.Tell(new DeadLetterCountResponse(_deadLetterCount)));
+    }
+
+    protected override void PreStart()
+    {
+        Context.System.EventStream.Subscribe(Self, typeof(DeadLetter));
+    }
+
+    protected override void PostStop()
+    {
+        Context.System.EventStream.Unsubscribe(Self, typeof(DeadLetter));
+    }
+}
+
+/// <summary>
+/// Message to request the current dead letter count.
+/// </summary>
+public sealed class GetDeadLetterCount
+{
+    public static readonly GetDeadLetterCount Instance = new();
+    private GetDeadLetterCount() { }
+}
+
+/// <summary>
+/// Response containing the current dead letter count.
+/// </summary>
+public sealed record DeadLetterCountResponse(long Count);