feat: Remove OpenRouter support, replace litellm with Chutes API

- Remove OPENROUTER from Provider enum, add CHUTES
- Update get_api_key() to use CHUTES_API_KEY
- Update get_base_url() to use https://api.chutes.ai/v1
- Rewrite LLM client using httpx instead of litellm
- Update default model to deepseek/deepseek-chat
- Remove litellm from requirements.txt and pyproject.toml
- Update all documentation references from OpenRouter/litellm to Chutes API
- Keep LiteLLMClient alias for backward compatibility