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

src/ScadaLink.Commons/Messages/Health/SiteHealthReport.cs

@@ -1,3 +1,4 @@
+using ScadaLink.Commons.Types;
 using ScadaLink.Commons.Types.Enums;
 
 namespace ScadaLink.Commons.Messages.Health;
@@ -32,7 +33,14 @@ public record SiteHealthReport(
     // marker). Surfaces a misconfigured / catastrophic regex on
     // /monitoring/health. Defaults to 0 for back-compat with existing
     // producers and tests that don't construct the field.
-    int AuditRedactionFailure = 0);
+    int AuditRedactionFailure = 0,
+    // Audit Log (#23) M6 Bundle E (T6): point-in-time snapshot of the
+    // site-local SQLite audit-log queue (pending count, oldest pending row,
+    // on-disk bytes). Populated by the site-side SiteAuditBacklogReporter
+    // hosted service every 30 s. Defaults to null so existing producers /
+    // tests that don't refresh the snapshot stay valid; the central health
+    // surface treats null as "no data yet" rather than a zeroed queue.
+    SiteAuditBacklogSnapshot? SiteAuditBacklog = null);
 
 /// <summary>
 /// Broadcast wrapper used between central nodes to keep per-node