v1.5.1: Gemini Guide Chat Backend

Summary

• POST /api/guide/chat powered by Google Gemini (gemini-3.1-flash-lite default)
• Loads secrets from .env via server/env.py (.env.example committed; .env gitignored)
• Python guide unit tests for Gemini payload helpers

Setup

Copy .env.example to .env and set GUIDE_AI_API_KEY from Google AI Studio.

Test plan

• pytest tests/test_guide.py passes
• Guide chat returns AI answers when key is configured