Add a `file` field to the `albumart` response indicating path of the artwork file used

[CamilleScholtz]

albumart currently works by scanning the directory of the target audio file for an image named cover.png, cover.jpg, or cover.webp. However, clients have no way to determine which specific file was actually used as the artwork source.

Exposing this information would be useful to clients for several reasons:

• Artwork comparison: Determine whether different tracks reference the same underlying artwork file, which helps avoid redundant processing.
• Effective caching: Use the artwork file path as a stable cache key instead of hashing artwork bytes or repeatedly requesting album art.
• Change detection: Easily detect when artwork has changed on disk (e.g., a user replaces cover.jpg), allowing clients to invalidate stale cache entries.
• Debugging and transparency: Provide developers and users with clearer insight into how artwork was resolved, which is especially helpful when multiple potential artwork files exist in the directory.

Request: Add a file field to the albumart response indicating path of the artwork file used.

Example output:

albumart foo/bar.ogg 0
file: foo/cover.png
size: 1024768
binary: 8192
<8192 bytes>
OK