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

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ScadaBridgeTelemetryTests.cs

@@ -24,6 +24,9 @@ public class ScadaBridgeTelemetryTests
             ScadaBridgeTelemetry.RecordInboundApiRequest("X");
             ScadaBridgeTelemetry.SiteConnectionOpened();
             ScadaBridgeTelemetry.SiteConnectionClosed();
+            ScadaBridgeTelemetry.LiveAlarmAggregatorStarted();
+            ScadaBridgeTelemetry.LiveAlarmAggregatorStopped();
+            ScadaBridgeTelemetry.RecordLiveAlarmStreamReconnect();
             ScadaBridgeTelemetry.SetQueueDepthProvider(() => 5);
         });