- Add chandra-ocr to fallback model list in api.ts
- Update Chat.tsx to detect models running outside model-manager
- Health check model endpoints to show externally managed models
- This allows Chandra OCR (running on spark-2 via custom vLLM build)
  to appear in the model dropdown

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>