- Add litellm to requirements.txt
- Add --parser CLI arg (auto/litellm, default auto)
- Create codai/litellm_backend.py module with:
  - LiteLLMBackend class for standardized responses
  - Rate limit headers (x-ratelimit-remaining-tokens, x-ratelimit-limit-tokens)
  - Qwen tool-call resilience (parse <tool> and <tool_call> tags)
  - Error handling with litellm exception mapping
- Update chat completions endpoint to use litellm when --parser litellm
- Update codai/__init__.py to export litellm components