refactor: rename EnableCoverArtUpload to EnableArtworkUpload

The config flag gates all image uploads (artists, radios, playlists),
not just cover art. Rename it to accurately reflect its scope across
the backend config, native API permission check, Subsonic CoverArtRole,
serve_index JSON key, and frontend config.

server/subsonic/users.go

@@ -22,7 +22,7 @@ func buildUserResponse(user model.User) responses.User {
 		ScrobblingEnabled: true,
 		DownloadRole:      conf.Server.EnableDownloads,
 		ShareRole:         conf.Server.EnableSharing,
-		CoverArtRole:      conf.Server.EnableCoverArtUpload || user.IsAdmin,
+		CoverArtRole:      conf.Server.EnableArtworkUpload || user.IsAdmin,
 		Folder:            slice.Map(user.Libraries, func(lib model.Library) int32 { return int32(lib.ID) }),
 	}