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/Health/DatabaseHealthCheck.cs

@@ -0,0 +1,34 @@
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+using ScadaLink.ConfigurationDatabase;
+
+namespace ScadaLink.Host.Health;
+
+/// <summary>
+/// Health check that verifies database connectivity for Central nodes.
+/// </summary>
+public class DatabaseHealthCheck : IHealthCheck
+{
+    private readonly ScadaLinkDbContext _dbContext;
+
+    public DatabaseHealthCheck(ScadaLinkDbContext dbContext)
+    {
+        _dbContext = dbContext;
+    }
+
+    public async Task<HealthCheckResult> CheckHealthAsync(
+        HealthCheckContext context,
+        CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            var canConnect = await _dbContext.Database.CanConnectAsync(cancellationToken);
+            return canConnect
+                ? HealthCheckResult.Healthy("Database connection is available.")
+                : HealthCheckResult.Unhealthy("Database connection failed.");
+        }
+        catch (Exception ex)
+        {
+            return HealthCheckResult.Unhealthy("Database connection failed.", ex);
+        }
+    }
+}