feat(ui): Audit KPI tiles on Health dashboard (#23 M7)

Adds three KPI tiles to the central Health dashboard for the Audit channel:
volume (rows in the last hour), error rate (Failed/Parked/Discarded over
total), and backlog (sum of SiteAuditBacklog.PendingCount across all sites).

Repo + service:
- IAuditLogRepository.GetKpiSnapshotAsync(window, nowUtc) — single aggregate
  SELECT over the trailing window returning total + error counts; nowUtc is
  optional for production callers and pinned by integration tests against the
  shared MSSQL fixture so the global counts are deterministic.
- AuditLogQueryService.GetKpiSnapshotAsync() — composes the repo aggregate
  with a sum of SiteAuditBacklog.PendingCount read from ICentralHealthAggregator.
- AuditLogKpiSnapshot record in Commons/Types/.

UI:
- New AuditKpiTiles Blazor component (Components/Health/) — three Bootstrap
  card-tiles, click navigates to /audit/log with the matching pre-filter.
- Health.razor wires the tiles in alongside the existing Notification Outbox
  KPIs; LoadAuditKpis() runs on every 10s refresh tick and degrades to em
  dashes + inline error if the query fails.
- AuditLogPage extended to parse ?status= so the error-rate tile drill-in
  (?status=Failed) auto-loads the grid.

Tests:
- AuditLogRepositoryTests: GetKpiSnapshotAsync mixed-status + empty-window
  cases against the MSSQL migration fixture.
- AuditLogQueryServiceTests: forwarding + backlog composition; sites with
  null SiteAuditBacklog contribute zero.
- AuditKpiTilesTests: 9 bUnit tests covering tile render, error-rate maths
  with safe zero-events handling, em-dash unavailable path, click-through
  navigation, and warning/danger border thresholds.
- HealthPageTests: new Renders_AuditKpiTiles_WithValues plus IAuditLogQueryService
  stub registration in the constructor so existing outbox tests still pass.
- AuditLogPageScaffoldTests: ?status=Failed auto-load + unknown status drop.

tests/ScadaLink.CentralUI.Tests/Pages/HealthPageTests.cs

@@ -6,9 +6,11 @@ using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 using NSubstitute;
+using ScadaLink.CentralUI.Services;
 using ScadaLink.Commons.Entities.Sites;
 using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.Commons.Messages.Notification;
+using ScadaLink.Commons.Types;
 using ScadaLink.Communication;
 using ScadaLink.HealthMonitoring;
 using HealthPage = ScadaLink.CentralUI.Components.Pages.Monitoring.Health;
@@ -55,6 +57,16 @@ public class HealthPageTests : BunitContext
             .Returns(Task.FromResult<IReadOnlyList<Site>>(new List<Site>()));
         Services.AddSingleton(siteRepo);
 
+        // Audit Log (#23) M7 Bundle E — the Health page now also fetches the
+        // Audit KPI snapshot. Stub it with an empty point-in-time reading so
+        // the existing assertions (Notification Outbox tiles, Online/Offline
+        // counts) keep passing; tests that target the Audit tiles set their
+        // own substitute.
+        var auditService = Substitute.For<IAuditLogQueryService>();
+        auditService.GetKpiSnapshotAsync(Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult(new AuditLogKpiSnapshot(0, 0, 0, DateTime.UtcNow)));
+        Services.AddSingleton(auditService);
+
         var claims = new[]
         {
             new Claim("Username", "tester"),
@@ -92,6 +104,35 @@ public class HealthPageTests : BunitContext
         Assert.Contains("View details", link.TextContent);
     }
 
+    [Fact]
+    public void Renders_AuditKpiTiles_WithValues()
+    {
+        // Override the default empty snapshot — this test wants concrete values
+        // to land in the three Audit tiles.
+        var auditService = Substitute.For<IAuditLogQueryService>();
+        auditService.GetKpiSnapshotAsync(Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult(new AuditLogKpiSnapshot(
+                TotalEventsLastHour: 250,
+                ErrorEventsLastHour: 5,
+                BacklogTotal: 17,
+                AsOfUtc: DateTime.UtcNow)));
+        Services.AddSingleton(auditService);
+
+        var cut = Render<HealthPage>();
+
+        cut.WaitForAssertion(() =>
+        {
+            // The three audit tiles render at the documented data-test selectors.
+            Assert.Contains("data-test=\"audit-kpi-volume\"", cut.Markup);
+            Assert.Contains("data-test=\"audit-kpi-error-rate\"", cut.Markup);
+            Assert.Contains("data-test=\"audit-kpi-backlog\"", cut.Markup);
+            // Volume shows the formatted thousand-separator value.
+            Assert.Contains("250", cut.Markup);
+            // Backlog renders 17.
+            Assert.Contains("17", cut.Markup);
+        });
+    }
+
     [Fact]
     public void OutboxKpiFailure_ShowsGracefulFallback()
     {