feat(sitestream): validate live-alarm-cache options + active-aggregator/reconnect telemetry (plan #10 T6)

Extend CommunicationOptionsValidator with eager bounds for the four T4
live-alarm-cache options (linger >= 0, reconcile > 0, seed concurrency 1..64,
subscribers-per-site >= 1). Enforce the per-site viewer cap fail-safe in
SiteAlarmLiveCacheService.Subscribe (reject excess viewers with a no-op
disposable rather than growing the list or throwing into the Blazor render
path). Surface two telemetry instruments on the existing ScadaBridgeTelemetry
meter: an active-aggregator observable gauge and a reconnect counter, wired from
the aggregator actor's PreStart/PostStop and its NodeA<->NodeB flip /
reconcile-driven reopen.

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.Communication/Actors/SiteAlarmAggregatorActor.cs

@@ -1,6 +1,7 @@
 using Akka.Actor;
 using Akka.Event;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.Streaming;
+using ZB.MOM.WW.ScadaBridge.Commons.Observability;
 using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
 
 namespace ZB.MOM.WW.ScadaBridge.Communication.Actors;
@@ -182,6 +183,9 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
     protected override void PreStart()
     {
         _log.Info("Starting site-alarm aggregator for site {0}", _siteIdentifier);
+        // Telemetry: this aggregator is now a running per-site live cache (gauge +1). Balanced
+        // in PostStop, which Akka always runs on termination for any reason.
+        ScadaBridgeTelemetry.LiveAlarmAggregatorStarted();
         _lifetimeCts = new CancellationTokenSource();
 
         // Stream-first: open the site-wide alarm stream BEFORE the first seed so deltas
@@ -204,6 +208,8 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         _lifetimeCts?.Cancel();
         _lifetimeCts?.Dispose();
         _lifetimeCts = null;
+        // Telemetry: this aggregator is no longer running (gauge -1). Balances PreStart.
+        ScadaBridgeTelemetry.LiveAlarmAggregatorStopped();
         base.PostStop();
     }
 
@@ -223,6 +229,8 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         {
             _log.Info("Site-alarm gRPC stream for {0} was down; reopening on reconcile tick", _siteIdentifier);
             _retryCount = 0;
+            // Telemetry: a reconcile-driven reopen after the stream was given up is a reconnect.
+            ScadaBridgeTelemetry.RecordLiveAlarmStreamReconnect();
             OpenGrpcStream();
         }
     }
@@ -455,6 +463,9 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         // Flip to the other node.
         _useNodeA = !_useNodeA;
 
+        // Telemetry: a NodeA↔NodeB failover flip is a reconnect + re-seed.
+        ScadaBridgeTelemetry.RecordLiveAlarmStreamReconnect();
+
         // A failover flip must RE-SEED (never silently serve stale) — kick a reconcile
         // fan-out alongside the reconnect. Buffering during the fan-out keeps the new
         // stream's deltas coherent with the fresh snapshot.