fix(core): add logs to trace subscribeGame call in createGame

Track whether subscribeGame is being called and completing successfully
to diagnose empty game-writeback messages.

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

modules/core/src/main/scala/de/nowchess/chess/resource/GameResource.scala

@@ -167,7 +167,9 @@ class GameResource:
     val mode  = req.mode.getOrElse(GameMode.Open)
     val entry = newEntry(GameContext.initial, white, black, tc, mode)
     registry.store(entry)
+    log.infof("About to subscribe game %s", entry.gameId)
     subscriberManager.subscribeGame(entry.gameId)
+    log.infof("Subscribed game %s", entry.gameId)
     log.infof(
       "Game %s created — white=%s black=%s mode=%s",
       entry.gameId,