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/CommunicationOptionsValidator.cs

@@ -65,5 +65,25 @@ public sealed class CommunicationOptionsValidator : OptionsValidatorBase<Communi
 
         builder.RequireThat(options.GrpcMaxConcurrentStreams > 0,
             $"Communication:GrpcMaxConcurrentStreams must be positive (was {options.GrpcMaxConcurrentStreams}).");
+
+        // ── Aggregated live alarm cache (plan #10, Task 6) ───────────────────────
+        // Linger drives a Timer dueTime; TimeSpan.Zero is valid (stop the aggregator
+        // immediately when the last viewer leaves), only a negative value is invalid.
+        builder.RequireThat(options.LiveAlarmCacheLinger >= TimeSpan.Zero,
+            $"Communication:LiveAlarmCacheLinger must be zero or a positive duration (was {options.LiveAlarmCacheLinger}).");
+
+        // Reconcile interval is a periodic timer cadence AND the start-retry cadence; a
+        // zero/negative value would spin or never fire.
+        builder.RequireThat(options.LiveAlarmCacheReconcileInterval > TimeSpan.Zero,
+            $"Communication:LiveAlarmCacheReconcileInterval must be a positive duration (was {options.LiveAlarmCacheReconcileInterval}).");
+
+        // Seed fan-out concurrency gates a SemaphoreSlim; at least 1, capped so a
+        // fat-fingered config can't open an unbounded burst of cross-cluster Asks.
+        builder.RequireThat(options.LiveAlarmCacheSeedConcurrency is >= 1 and <= 64,
+            $"Communication:LiveAlarmCacheSeedConcurrency must be between 1 and 64 (was {options.LiveAlarmCacheSeedConcurrency}).");
+
+        // Per-site viewer cap must admit at least one viewer, else the page could never go live.
+        builder.RequireThat(options.LiveAlarmCacheMaxSubscribersPerSite >= 1,
+            $"Communication:LiveAlarmCacheMaxSubscribersPerSite must be at least 1 (was {options.LiveAlarmCacheMaxSubscribersPerSite}).");
     }
 }