Chess server is to be horizontally scaled

[khemssharma]

Summary

Matchmaking queue and live Game/StockfishGame state live only in the
memory of a single Node process. Redis is used only for reconnect persistence (surviving a
process restart), not for routing live traffic between instances.

Impact

Running more than one server instance behind a load balancer will silently
break active games: a player matched on instance A can't receive a move
from a player connected to instance B. This fails quietly rather than
erroring loudly, which makes it a scaling trap rather than a scaling wall.

Proposed direction (needs discussion before implementation)

• Option A: Redis pub/sub to broadcast moves across instances, with game
  state read from/written to Redis as source of truth instead of in-process
  objects.
• Option B: sticky sessions at the load balancer + a dedicated
  matchmaking service that hands off a (gameId, instanceId) pair, so a
  given game always routes to the instance that owns it.

This is a meaningfully bigger change and touches matchmaking,
reconnect, and disconnect-handling logic, so I'm opening this as a design
discussion rather than shipping a fix directly.