UI minor - audio album image

how to select the image for the album.

According to Tracey, the heuristic is complicated, with lots of edge cases, that makes it problematic to duplicate (and keep maintained).

Solution...
* Tracey to add a service e.g. `services/imgurl/foo` or services/img/foo?output=name` that returns the name of the file, rather than the actual image
* Gateway to retrieve this name, then check the file metadata.
* For OK files, the image is added to the dweb, and its url's returned in a metadata field
* For Non-OK files, the `services/img/foo?scale=2` (syntax TBD) call is used to get a larger file which is added to the Dweb and urls' returned.

Non-OK means any of:
file/private is set - so only accessable to priviliged users
file/format us unusable e.g. BMP, TIF, PDF etc
file/size is too large (above a TBD amount)

All the details here are fuzzy, but give the idea of how to solve the problem.

Start by clarifying the album-image heuristic and the contract between the proposed services/imgurl/foo or services/img/foo endpoint and the Gateway. Define the metadata checks, fallback scaling behavior, and URL fields before implementation; done means acceptable images are added to the dweb and non-OK images use the agreed fallback.

Written by the indexing model from the issue text.