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

packages/db/src/generating-deck.ts

@@ -103,7 +103,7 @@ const createDeck = async (tx: DbOrTx, validatedLanguages: string[]) => {
       description: config.deckDescription,
       source_language: config.sourceLanguage,
       validated_languages: validatedLanguages,
-      is_public: false,
+      type: "core",
     })
     .returning({ id: decks.id });
   const created = result[0];