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.Commons/Types/AuditLogKpiSnapshot.cs

@@ -0,0 +1,38 @@
+namespace ScadaLink.Commons.Types;
+
+/// <summary>
+/// Audit Log (#23) M7 Bundle E (T13) — point-in-time KPI snapshot for the central
+/// Health dashboard's "Audit" tile group. Aggregates volume + error counts over
+/// the trailing window from the central <c>AuditLog</c> table and combines them
+/// with the global pending backlog summed across every site's
+/// <see cref="SiteAuditBacklogSnapshot"/>.
+/// </summary>
+/// <param name="TotalEventsLastHour">
+/// Total <c>AuditLog</c> rows whose <c>OccurredAtUtc</c> falls inside the trailing
+/// 1-hour window. Drives the "Audit volume" tile and the denominator of
+/// "Audit error rate". A zero value renders as "0" rather than an em dash —
+/// "zero rows in the last hour" is a real, valid signal in a quiet system.
+/// </param>
+/// <param name="ErrorEventsLastHour">
+/// Total <c>AuditLog</c> rows in the same window whose <see cref="Enums.AuditStatus"/>
+/// is <c>Failed</c>, <c>Parked</c>, or <c>Discarded</c>. Drives the "Audit error
+/// rate" tile numerator; clicking the tile drills in to <c>/audit/log</c>
+/// pre-filtered on one of those statuses.
+/// </param>
+/// <param name="BacklogTotal">
+/// Sum of <c>SiteAuditBacklog.PendingCount</c> across every site's latest
+/// <see cref="ScadaLink.Commons.Messages.Health.SiteHealthReport"/>. Sites whose
+/// snapshot is <c>null</c> (no report yet, or reporter not running) contribute
+/// zero. A persistently non-zero value across multiple refresh ticks indicates
+/// the site→central drain isn't keeping up.
+/// </param>
+/// <param name="AsOfUtc">
+/// UTC timestamp at which the snapshot was computed. Used by the UI to label
+/// "as of HH:mm:ss" beneath the tile group and to detect stale data when a
+/// refresh tick fails.
+/// </param>
+public sealed record AuditLogKpiSnapshot(
+    long TotalEventsLastHour,
+    long ErrorEventsLastHour,
+    long BacklogTotal,
+    DateTime AsOfUtc);