Add URL_PREFIX for fastapi root_path
This should address the issues we are observing on production when it references static file that do not exist.
When FastAPI is behind a proxy (as with this deployment) you need to specify a root_path kwarg to allow fastapi to properly serve static files, handle reroutes, etc. This add conditional check for URL_PREFIX env var and adds if specified. https://fastapi.tiangolo.com/advanced/behind-a-proxy/