feat(quizzes): integrate chainlearn-ai service for quiz generation (closes #3)

[Labi-Joy]

closes #3

generateQuiz now calls the chainlearn-ai service (POST /generate-quiz) instead of returning the hardcoded Stellar questions. If the AI service is unreachable, it falls back to the existing placeholder set so the dashboard doesn't break on a transient outage.

what changed:

• AI_SERVICE_URL env var added to the zod schema, defaults to http://localhost:8000
• new src/modules/quizzes/ai-client.ts wrapping the AI service call
• generateQuizSchema accepts optional difficulty and numQuestions (defaulted to beginner / 5 in the service)
• placeholder generator is kept as the fallback path

testing:

• npm run typecheck clean
• npm run lint clean on touched files (existing warnings on other files untouched)
• npm test passes (36/36)
• manual: start chainlearn-ai locally, hit POST /api/quizzes/generate, confirm questions come from the AI service; stop the AI service and confirm the fallback kicks in with a warn log

[DeFiVC]

Clean implementation that fully addresses issue #3. The AI service integration with graceful fallback is well-structured.

What's Good

• Proper separation: ai-client.ts encapsulates the AI service call cleanly, keeping quiz.service.ts focused on business logic.
• Graceful fallback: If the AI service is unreachable, placeholder questions are returned with a warn log — the dashboard never breaks.
• Type safety: AiDifficulty enum and GenerateQuizFromAIParams interface exported from the client for consumers.
• Schema updates: difficulty and numQuestions added as optional fields with proper Zod validation.
• Field mapping: AI response (prompt → text, minor → correctIndex) mapped correctly to the existing QuizQuestion format.

Minor Observations (non-blocking)

• config.AI_SERVICE_URL uses uppercase with underscores (env var convention) — good choice over the camelCase suggested in the issue.
• The AiQuizResponse.quiz_id field from the AI service isn't stored locally — this is fine since the API generates its own quiz ID.

CI green, no conflicts, linked issue matched. Approving.