Add tailored AI hints for code review challenges

[Zhu-E]

Summary

• add Gemma-powered tailored hints for code review challenges
• wire the code review UI to request adaptive hints based on the learner's current patch
• add fallback/backend test coverage for progressive code review hints

Testing

• npm run build
• pytest backend/tests/test_ai_hints.py -q (blocked locally: fastapi not installed in this environment)