feat(audit-log): surface partition-purge failure as health event + counter

src/ZB.MOM.WW.ScadaBridge.AuditLog/ServiceCollectionExtensions.cs

@@ -212,6 +212,12 @@ public static class ServiceCollectionExtensions
         // redaction-failure counters.
         services.TryAddSingleton<IAuditInboundCeilingHitsCounter, NoOpAuditInboundCeilingHitsCounter>();
 
+        // AuditLog purge-failure counter — NoOp default for site/test
+        // composition roots. AddAuditLogCentralMaintenance below replaces this binding
+        // with the AuditCentralHealthSnapshot implementation so partition/channel purge
+        // failures surface on the central dashboard alongside the write-failure counter.
+        services.TryAddSingleton<IAuditPurgeFailureCounter, NoOpAuditPurgeFailureCounter>();
+
         // Central direct-write audit writer used by
         // NotificationOutboxActor and Inbound API to
         // emit AuditLog rows that originate ON central, not via site telemetry.
@@ -419,6 +425,11 @@ public static class ServiceCollectionExtensions
         // ICentralAuditWriteFailureCounter above.
         services.Replace(ServiceDescriptor.Singleton<IAuditInboundCeilingHitsCounter>(
             sp => sp.GetRequiredService<AuditCentralHealthSnapshot>()));
+        // Replace the NoOp IAuditPurgeFailureCounter with the AuditCentralHealthSnapshot so
+        // partition/channel purge failures surface on the central dashboard. Same
+        // singleton-forward pattern as ICentralAuditWriteFailureCounter above (arch-review 04, S2).
+        services.Replace(ServiceDescriptor.Singleton<IAuditPurgeFailureCounter>(
+            sp => sp.GetRequiredService<AuditCentralHealthSnapshot>()));
 
         return services;
     }