feat(health): SiteAuditBacklog metric (count + age + bytes) (#23 M6)

src/ScadaLink.Commons/Types/SiteAuditBacklogSnapshot.cs

@@ -0,0 +1,32 @@
+namespace ScadaLink.Commons.Types;
+
+/// <summary>
+/// Audit Log (#23) M6 Bundle E (T6) — point-in-time snapshot of the site-local
+/// SQLite audit-log queue health, surfaced on
+/// <see cref="ScadaLink.Commons.Messages.Health.SiteHealthReport"/> as
+/// <c>SiteAuditBacklog</c> and refreshed periodically by the
+/// <c>SiteAuditBacklogReporter</c> hosted service.
+/// </summary>
+/// <param name="PendingCount">
+/// Number of rows currently in
+/// <see cref="ScadaLink.Commons.Types.Enums.AuditForwardState.Pending"/> — i.e.
+/// not yet acknowledged by central via either the push-telemetry or
+/// reconciliation-pull paths. A persistently non-zero value with rising
+/// <see cref="OldestPendingUtc"/> indicates the site→central drain isn't
+/// keeping up.
+/// </param>
+/// <param name="OldestPendingUtc">
+/// <see cref="ScadaLink.Commons.Entities.Audit.AuditEvent.OccurredAtUtc"/> of
+/// the oldest Pending row, or <c>null</c> if the queue is empty. Used by ops
+/// to compute backlog age without a separate query.
+/// </param>
+/// <param name="OnDiskBytes">
+/// Size of the SQLite file on disk in bytes, or <c>0</c> if the writer is
+/// running against an in-memory database. Mirrors the 7-day retention
+/// invariant (alog.md §10) — a steady file-size growth past the retention
+/// window points at a stuck purge or a stuck forwarder.
+/// </param>
+public sealed record SiteAuditBacklogSnapshot(
+    int PendingCount,
+    DateTime? OldestPendingUtc,
+    long OnDiskBytes);