feat(lmxproxy): add delivered/dropped message counts to subscription stats

Subscription metrics (totalDelivered, totalDropped) now visible in
/api/status JSON and HTML dashboard. Card turns yellow if drops > 0.
Aggregated from per-client counters in SubscriptionManager.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lmxproxy/src/ZB.MOM.WW.LmxProxy.Host/Status/StatusModels.cs

@@ -28,6 +28,8 @@ namespace ZB.MOM.WW.LmxProxy.Host.Status
         public int TotalClients { get; set; }
         public int TotalTags { get; set; }
         public int ActiveSubscriptions { get; set; }
+        public long TotalDelivered { get; set; }
+        public long TotalDropped { get; set; }
     }
 
     public class PerformanceStatus