feat: multi-provider LLM support (OpenAI, Gemini)

Add protocol-based LLM adapter (providers.py) supporting Anthropic,
OpenAI, and Google Gemini with a unified interface. Reviewer is now
fully provider-agnostic. API endpoint accepts provider/model/key
overrides per request.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>