Add OpenAI fallback: gpt-* LLM_MODEL names route to ChatOpenAI

Production safety net for the Fable 5 cutover: flipping LLM_MODEL to a
gpt-* name in the Render dashboard falls back to OpenAI without a code
change. OPENAI_API_KEY re-declared in render.yaml (sync: false) so the
existing dashboard value survives blueprint sync.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>