Add the session storage infrastructure for tracking correct answers
during a game. Designed for easy swap to Valkey in Phase 4.
- GameSessionStore interface with create/get/delete methods, all async
to match the eventual Valkey implementation
- InMemoryGameSessionStore backed by a Map
- GameSessionData holds only the answer key (questionId → correctOptionId)
- Also fix root build script to build packages in dependency order
- Add GameRequestSchema and derived types to packages/shared
- Add SupportedLanguageCode, SupportedPos, DifficultyLevel type exports
- Add getGameTerms() model to packages/db with pos/language/difficulty/limit filters
- Add prepareGameQuestions() service skeleton in apps/api
- Add createGame controller with Zod safeParse validation
- Wire POST /api/v1/game/start route
- Add scripts/gametest/test-game.ts for manual end-to-end testing
