Add MiniMax AI as the 7th LLM provider, supporting chat (M2.7, M2.5,
M2.5-highspeed) and embedding (embo-01) models via OpenAI-compatible API.
Changes:
- Add MINIMAX enum to ChatModeType
- Add MinimaxServiceImpl chat provider (OpenAI-compat streaming)
- Add MinimaxEmbeddingProvider for vector embeddings
- Add SQL migration for provider and model registration
- Add 14 unit tests + 3 integration tests
- Update README/README_EN with MiniMax in provider list