docs: Add Groq API and Kimi2 model documentation to README

[nobita2041]

Summary

• Document the actual AI models used in the codebase
• Add GROQ_API_KEY to environment variables setup
• Explain the AI provider fallback mechanism

Changes

• Added GROQ_API_KEY to the environment variables section
• Created new "AI Models" section documenting:
  • Groq API as primary provider using moonshotai/kimi-k2-instruct model
  • OpenAI API as fallback option
  • Automatic fallback to mock data when both APIs are unavailable

Test plan

• Verify README accurately reflects the codebase implementation
• Confirm all required environment variables are documented
• Check that the AI provider priority is correctly described

🤖 Generated with Claude Code