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

src/ScadaLink.HealthMonitoring/ISiteHealthCollector.cs

@@ -1,4 +1,5 @@
 using ScadaLink.Commons.Messages.Health;
+using ScadaLink.Commons.Types;
 using ScadaLink.Commons.Types.Enums;
 
 namespace ScadaLink.HealthMonitoring;
@@ -28,6 +29,15 @@ public interface ISiteHealthCollector
     /// <c>AddAuditLogHealthMetricsBridge()</c>.
     /// </summary>
     void IncrementAuditRedactionFailure();
+    /// <summary>
+    /// Audit Log (#23) M6 Bundle E (T6) — replace the latest site-local
+    /// audit-queue backlog snapshot (pending count, oldest pending row,
+    /// on-disk file bytes) used by the next <see cref="CollectReport"/> call.
+    /// Refreshed periodically by the <c>SiteAuditBacklogReporter</c> hosted
+    /// service so each report carries a recent point-in-time view of the
+    /// site→central drain health.
+    /// </summary>
+    void UpdateSiteAuditBacklog(SiteAuditBacklogSnapshot snapshot);
     void UpdateConnectionHealth(string connectionName, ConnectionHealth health);
     void RemoveConnection(string connectionName);
     void UpdateTagResolution(string connectionName, int totalSubscribed, int successfullyResolved);