The FastAPI backend must use FastAPI's automatic OpenAPI generation as the source of truth for API documentation.

Requirements:
- define request and response models properly with Pydantic
- add endpoint summaries and descriptions
- add tags for endpoint grouping
- add example payloads where useful
- expose the generated OpenAPI schema
- keep Swagger/ReDoc available for internal use if helpful

On top of the generated OpenAPI spec, provide a better user-facing documentation experience for customers, with:
- polished API reference
- quickstart guides
- authentication guide
- end-to-end examples
- code samples in multiple languages