Allow administrators to disable playlist cover art upload/removal for
non-admin users via the new EnableCoverArtUpload config option (default: true).
- Guard uploadPlaylistImage and deletePlaylistImage endpoints (403 for non-admin when disabled)
- Set CoverArtRole in Subsonic GetUser/GetUsers responses based on config and admin status
- Pass config to frontend and conditionally hide upload/remove UI controls
- Admins always retain upload capability regardless of setting