Embedding art into singletons using `cover_art_url` field

### Proposed solution

Currently, art gets automatically embedded to albums. However, this doesn't seem to apply to singletons.

### Objective

Automatically embed art into tracks / singletons using `cover_art_url` field, if available.

The issue does not name files, tests, or an entry point. Start by locating the existing album art embedding flow and how singleton tracks are processed, then trace where cover_art_url is read. Done means tracks or singletons with an available cover_art_url have their art embedded, with coverage for that behavior.

Written by the indexing model from the issue text.