perf(router): replace 100ms Valkey polling with Pub/Sub notification wakeup and increase heartbeat to 45s

The Valkey transport layer used 100ms busy-polling loops (Task.Delay(100))
across ~90 concurrent loops in 45+ services, generating ~900 idle
commands/sec and burning ~58% CPU while the system was completely idle.

Replace polling with Redis Pub/Sub notifications:
- Publishers fire PUBLISH after each XADD (fire-and-forget)
- Consumers SUBSCRIBE and wait on SemaphoreSlim with 30s fallback timeout
- Applies to both ValkeyMessageQueue (INotifiableQueue) and ValkeyEventStream
- Non-Valkey transports fall back to 1s polling via QueueWaitExtensions

Increase heartbeat interval from 10s to 45s across all transport options,
with corresponding health threshold adjustments (stale: 135s, degraded: 90s).

Expected idle CPU reduction: ~58% → ~3-5%.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Router/__Libraries/StellaOps.Router.Gateway/Configuration/HealthOptions.cs

@@ -12,21 +12,23 @@ public sealed class HealthOptions
 
     /// <summary>
     /// Gets or sets the threshold after which a connection is considered stale (no heartbeat).
-    /// Default: 30 seconds.
+    /// Should be at least 3x the heartbeat interval (45s default).
+    /// Default: 135 seconds.
     /// </summary>
-    public TimeSpan StaleThreshold { get; set; } = TimeSpan.FromSeconds(30);
+    public TimeSpan StaleThreshold { get; set; } = TimeSpan.FromSeconds(135);
 
     /// <summary>
     /// Gets or sets the threshold after which a connection is considered degraded.
-    /// Default: 15 seconds.
+    /// Should be at least 2x the heartbeat interval (45s default).
+    /// Default: 90 seconds.
     /// </summary>
-    public TimeSpan DegradedThreshold { get; set; } = TimeSpan.FromSeconds(15);
+    public TimeSpan DegradedThreshold { get; set; } = TimeSpan.FromSeconds(90);
 
     /// <summary>
     /// Gets or sets the interval at which to check for stale connections.
-    /// Default: 5 seconds.
+    /// Default: 15 seconds.
     /// </summary>
-    public TimeSpan CheckInterval { get; set; } = TimeSpan.FromSeconds(5);
+    public TimeSpan CheckInterval { get; set; } = TimeSpan.FromSeconds(15);
 
     /// <summary>
     /// Gets or sets the number of ping measurements to keep for averaging.