feat(shared): add quiz session Zod schemas

Add the shared schemas for the quiz request/response cycle, defining
the contract between the API and the frontend.

- Reorganise packages/shared: move schemas into a schemas/ subdirectory
  grouped by domain. Delete the old flat schema.ts.
- Add AnswerOption, GameQuestion, GameSession, AnswerSubmission, and
  AnswerResult alongside the existing GameRequest.
- optionId is an integer 0-3 (positional, shuffled at session-build
  time so position carries no information).
- questionId and sessionId are UUIDs (globally unique, opaque, natural
  keys for Valkey storage later).
- gloss is  rather than optional, for a predictable
  shape on the frontend.
- options array enforced to exactly 4 elements at the schema level.

packages/shared/src/index.ts

@@ -1,2 +1,2 @@
 export * from "./constants.js";
-export * from "./schema.js";
+export * from "./schemas/game.js";