(SP: 1) [Backend] Redis Cache for Quiz Questions #291

Goal

Eliminate 2 DB queries per quiz page load by caching quiz questions in Redis.

Scope

• Create types/quiz.ts with shared quiz types (QuizQuestion, QuizAnswer, QuizQuestionWithAnswers)
• Add getOrCreateQuestionsCache(quizId, locale) to lib/quiz/quiz-answers-redis.ts
• Integrate cache into getQuizQuestions() in db/queries/quiz.ts
• Cache key: quiz:questions:{quizId}:{locale}
• Fallback to DB if Redis not configured

Expected impact

• Performance: ~5x faster quiz page load on cache hit (354ms → 65ms)
• DB load: 2 fewer queries per quiz page view
• Scalability: Popular quizzes served from Redis, not DB

Out of scope

• TTL configuration (cache persists until manual clear or Redis eviction)
• Cache invalidation on quiz content update (future admin feature)
• Leaderboard caching (users want instant rank updates)

Files changed

• frontend/types/quiz.ts (new)
• frontend/lib/quiz/quiz-answers-redis.ts (modified)
• frontend/db/queries/quiz.ts (modified)