feat: add coordinator startup validation and K8s pod watch

On startup, load all known instances from Redis and wait 15s for them to
reconnect via gRPC. Evict instances that don't reconnect within the timeout
and delete their K8s pods.

Replace one-shot pod status check with real fabric8 Watch. On pod Terminating
event, mark instance dead. On pod Deleted event, trigger failover. Failover
now waits reactively for at least one healthy instance before distributing
orphaned games, up to 30s timeout.

- Add startupValidationTimeout and failoverWaitTimeout config (15s, 30s)
- CoordinatorGrpcServer tracks active gRPC streams
- InstanceRegistry.loadAllFromRedis() scans and loads instances on startup
- HealthMonitor startup observer validates instances and starts K8s watch
- FailoverService.onInstanceStreamDropped returns Uni[Unit] for reactive wait
- All failover service callers updated to subscribe to Uni result

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

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

@@ -45,6 +45,8 @@ nowchess:
     k8s-namespace: default
     k8s-rollout-name: nowchess-core
     k8s-rollout-label-selector: "app=nowchess-core"
+    startup-validation-timeout: 15s
+    failover-wait-timeout: 30s
 
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