Feat/in memory cache

[xrvnd]

Summary

refactor: unify board cache using node-cache with (date+boardSize) key

• Replaced two manual Maps (cachedBoardsByKey + cachedBoardsById) with setTimeout eviction with a node-cache instance
• Unified all board access through a single cache key: date:boardSize changed from the puzzleId lookup path entirely
• Updated all callers to pass (date + boardSize) instead of puzzleId, including API routes and client-side game actions.
• Cache TTL is set to seconds until UTC midnight (end of day), after which node-cache evicts the entry automatically

Test

I manually verified using temporary console.log("CACHE HIT") / console.log("CACHE MISS") logs:

• first request results in CACHE MISS + DB fetch
• subsequent hint/check requests result in CACHE HIT without additional DB queries

Closes coder-zs-cse#11