Retrieve File

The bytes behind a file id. The metadata route answers what a file is; this one answers with the file. An agent that produces a document, an image or a crawl reports an id, and without this route that id names bytes the caller cannot read. The response carries the stored mime type and a `Content-Disposition` of `attachment`, so a browser opening the URL saves the file rather than rendering whatever the bytes happen to be. NOTE: every docstring on a route in this module is **published**. The docs site generates a page per operation from the OpenAPI spec, and `docs-site/scripts/check-content.mjs` holds those pages to house style, so an em dash here fails the docs build rather than review.