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

src/ScadaLink.Commons/Interfaces/Services/ISiteAuditQueue.cs

@@ -1,4 +1,5 @@
 using ScadaLink.Commons.Entities.Audit;
+using ScadaLink.Commons.Types;
 
 namespace ScadaLink.Commons.Interfaces.Services;
 
@@ -70,4 +71,17 @@ public interface ISiteAuditQueue
     /// are left untouched (idempotent re-call). Non-existent ids are silent no-ops.
     /// </summary>
     Task MarkReconciledAsync(IReadOnlyList<Guid> eventIds, CancellationToken ct = default);
+
+    /// <summary>
+    /// M6 Bundle E (T6) health-metric surface: returns a point-in-time snapshot
+    /// of the site queue's pending count + oldest pending timestamp + on-disk
+    /// SQLite file size. Surfaced on
+    /// <see cref="ScadaLink.Commons.Messages.Health.SiteHealthReport"/> as
+    /// <c>SiteAuditBacklog</c> by the periodic <c>SiteAuditBacklogReporter</c>
+    /// hosted service so a stuck site→central drain is visible on the central
+    /// health dashboard. Safe to call concurrently with hot-path writes —
+    /// implementations are expected to take the same connection lock used by
+    /// the hot-path INSERT batch and the drain queries.
+    /// </summary>
+    Task<SiteAuditBacklogSnapshot> GetBacklogStatsAsync(CancellationToken ct = default);
 }