feat(api): wire GameSessionStore into createGameSession

The service now tracks the correct optionId for each question and
stores the answer key in the GameSessionStore after building the
session. The client response is unchanged — the store is invisible
to the outside.

- Build answerKey (questionId → correctOptionId) during question
  assembly by finding the correct answer's position after shuffle
- Store the answer key via gameSessionStore.create() before returning
- Add excludeText parameter to getDistractors to prevent a distractor
  from having identical text to the correct answer (different term,
  same translation). Solved at the query level, not with post-filtering.
- Module-level InMemoryGameSessionStore singleton in the service

apps/api/src/services/gameService.ts

@@ -22,6 +22,7 @@ export const createGameSession = async (
     correctAnswers.map(async (correctAnswer) => {
       const distractorTexts = await getDistractors(
         correctAnswer.termId,
+        correctAnswer.targetText,
         request.target_language,
         request.pos,
         request.difficulty,