feat(api): add in-memory GameSessionStore

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

package.json

@@ -4,6 +4,7 @@
   "description": "a vocabulary trainer",
   "private": true,
   "scripts": {
+    "build": "pnpm --filter @glossa/shared build && pnpm --filter @glossa/db build",
     "dev": "concurrently --names \"api,web\" -c \"magenta.bold,green.bold\" \"pnpm --filter @glossa/api dev\" \"pnpm --filter @glossa/web dev\"",
     "test": "vitest",
     "test:run": "vitest run",