feat: scale up on high CPU load, not just subscription count

AutoScaler now checks K8s pod metrics (CPU) in addition to subscription count.
Scale-up triggers if:
1. avgLoad > scaleUpThreshold * maxGamesPerCore, OR
2. Any instance has CPU > 800m

Fixes scenario where instance under heavy CPU load wouldn't scale without
high subscription count. Now responds to compute utilization, not just game count.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

modules/coordinator/src/main/resources/application.yml

@@ -28,7 +28,7 @@ nowchess:
   internal:
     secret: ${INTERNAL_SECRET:123abc}
   coordinator:
-    max-games-per-core: 500
+    max-games-per-core: 100
     max-deviation-percent: 20
     rebalance-interval: 30s
     rebalance-min-interval: 60s