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

src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs

@@ -218,6 +218,14 @@ public static class ServiceCollectionExtensions
             ServiceDescriptor.Singleton<IAuditWriteFailureCounter, HealthMetricsAuditWriteFailureCounter>());
         services.Replace(
             ServiceDescriptor.Singleton<IAuditRedactionFailureCounter, HealthMetricsAuditRedactionFailureCounter>());
+        // M6 Bundle E (T6): the site-side backlog reporter polls the
+        // SqliteAuditWriter every 30 s and pushes the snapshot into the
+        // collector so the next SiteHealthReport carries a fresh
+        // SiteAuditBacklog field. Registered alongside the other site-only
+        // metric bridges so AddAuditLog (which runs on central too) stays
+        // free of hosted-service registrations that would resolve a missing
+        // ISiteHealthCollector on central.
+        services.AddHostedService<SiteAuditBacklogReporter>();
         return services;
     }