Fetch book cover images with the Open Library Covers API

Steps

1. Build an image URL of the form https://covers.openlibrary.org/b/isbn/{ISBN}-{SIZE}.jpg, where SIZE is S, M, or L.
2. Alternatively key off an Open Library ID (olid) or internal cover ID using the same /b/{key}/{value}-{size}.jpg pattern.
3. Request the size appropriate to the surface (S for thumbnails/lists, L for detail pages) to avoid unnecessary bandwidth.
4. Handle the case where no cover exists -- the API can return a small placeholder image rather than an HTTP error, so check response dimensions/content-length.
5. No authentication is required for basic cover fetches.