Proposal: Add configurable download subdirectory templating

[paubineau]

My goal is to allow Explo downloads to use a media-server-friendly folder layout, such as {artist}/{album}, so the Explo collection displays correctly in Jellyfin while still supporting non-persistent playlists.

• Add DOWNLOAD_SUBDIRECTORY_FORMAT to customize download subfolders when USE_SUBDIRECTORY=true
• Support {playlist}, {artist}, and {album} tokens, with {playlist} preserving the existing default behavior
• Keep flat downloads unchanged when USE_SUBDIRECTORY=false
• Expose the new subdirectory format option in the web settings and setup wizard
• Add file based (called playlist-manifest) tracking/cleanup for non-persistent runs using formats without a {playlist} root
• Remove empty artist/album folders when playlist-manifest cleanup deletes unreferenced files
• Refresh Jellyfin images during library scans
• Write album_artist metadata for YouTube downloads to improve media server grouping

Notes:

DOWNLOAD_SUBDIRECTORY_FORMAT is ignored unless USE_SUBDIRECTORY=true.

Formats rooted by {playlist} keep the existing cleanup behavior: non-persistent runs delete that playlist's download folder.

For formats that do not start with {playlist}, Explo cannot safely delete an entire playlist folder anymore because files from different playlists may share the same artist/album folders. In that case, Explo stores a per-playlist cleanup record under playlist-manifests and only removes files that are no longer referenced by another Explo playlist.

The cleanup implementation is split into path/template helpers (paths.go) and playlist-manifest helpers (manifest.go): path helpers resolve the configured subdirectory format safely, while playlist manifests track which downloaded files belong to each playlist when multiple playlists share the same artist/album folders.

Playlist cleanup only tracks files created by Explo during a run, or files already known from another Explo playlist manifest. Existing user-library files are not claimed for cleanup.

Possible split:

If this PR is not something you want to merge because of the added complexity, the Jellyfin-related changes may still be worth considering separately:

• Refresh Jellyfin images during library scans
• Write album_artist metadata for YouTube downloads to improve media server grouping

These changes are independent from the configurable subdirectory feature and improve how Explo-downloaded tracks are grouped and displayed with artwork in Jellyfin.

Disclosure:

I used AI assistance (Codex) to speed up implementation while preparing this PR. I reviewed and tested all changes before submitting.