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.

src/ScadaLink.CentralUI/Services/IAuditLogQueryService.cs

@@ -1,4 +1,5 @@
 using ScadaLink.Commons.Entities.Audit;
+using ScadaLink.Commons.Types;
 using ScadaLink.Commons.Types.Audit;
 
 namespace ScadaLink.CentralUI.Services;
@@ -27,4 +28,26 @@ public interface IAuditLogQueryService
 
     /// <summary>Default page size when callers don't specify one.</summary>
     int DefaultPageSize { get; }
+
+    /// <summary>
+    /// Audit Log (#23) M7 Bundle E (T13) — returns the point-in-time KPI snapshot
+    /// the Health dashboard's Audit tiles render. Composes:
+    /// <list type="bullet">
+    ///   <item><c>TotalEventsLastHour</c> + <c>ErrorEventsLastHour</c> from
+    ///   <see cref="ScadaLink.Commons.Interfaces.Repositories.IAuditLogRepository.GetKpiSnapshotAsync"/>
+    ///   (1-hour trailing window).</item>
+    ///   <item><c>BacklogTotal</c> from the sum of every site's
+    ///   <c>SiteHealthReport.SiteAuditBacklog.PendingCount</c> via
+    ///   <see cref="ScadaLink.HealthMonitoring.ICentralHealthAggregator"/>.</item>
+    /// </list>
+    /// </summary>
+    /// <remarks>
+    /// Repository + aggregator are read independently; if either source has no
+    /// data the corresponding field is zero (a real signal — "no events" vs
+    /// "no backlog" — rather than an error). The service does NOT swallow
+    /// exceptions; the page wraps the call in a try/catch so a transient DB
+    /// outage degrades the tile group to "unavailable" rather than killing the
+    /// dashboard.
+    /// </remarks>
+    Task<AuditLogKpiSnapshot> GetKpiSnapshotAsync(CancellationToken ct = default);
 }