feat: add MiniMax as LLM provider

Add MiniMax (https://platform.minimaxi.com) as a new LLM provider option.
MiniMax API is OpenAI-compatible and supports models like MiniMax-M1.

Changes:
- Add minimax provider branch in app/services/llm.py
- Add minimax configuration in config.example.toml
- Update both README.md and README-en.md to list MiniMax