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

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/AuditCentralHealthSnapshot.cs

@@ -40,11 +40,13 @@ public sealed class AuditCentralHealthSnapshot
     : IAuditCentralHealthSnapshot,
       ICentralAuditWriteFailureCounter,
       IAuditRedactionFailureCounter,
-      IAuditInboundCeilingHitsCounter
+      IAuditInboundCeilingHitsCounter,
+      IAuditPurgeFailureCounter
 {
     private int _centralAuditWriteFailures;
     private int _auditRedactionFailure;
     private int _auditInboundCeilingHits;
+    private int _auditPurgeFailures;
     private readonly ConcurrentDictionary<string, bool> _stalled = new();
 
     /// <inheritdoc/>
@@ -59,6 +61,10 @@ public sealed class AuditCentralHealthSnapshot
     public int AuditInboundCeilingHits =>
         Interlocked.CompareExchange(ref _auditInboundCeilingHits, 0, 0);
 
+    /// <inheritdoc/>
+    public int PurgeFailures =>
+        Interlocked.CompareExchange(ref _auditPurgeFailures, 0, 0);
+
     /// <inheritdoc/>
     public IReadOnlyDictionary<string, bool> SiteAuditTelemetryStalled =>
         new Dictionary<string, bool>(_stalled);
@@ -88,4 +94,8 @@ public sealed class AuditCentralHealthSnapshot
     /// <inheritdoc/>
     void IAuditInboundCeilingHitsCounter.Increment() =>
         Interlocked.Increment(ref _auditInboundCeilingHits);
+
+    /// <inheritdoc/>
+    void IAuditPurgeFailureCounter.Increment() =>
+        Interlocked.Increment(ref _auditPurgeFailures);
 }