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/nativeapi/image_upload.go

@@ -24,8 +24,8 @@ const maxImageSize = 10 << 20 // 10MB
 
 func checkImageUploadPermission(w http.ResponseWriter, r *http.Request) bool {
 	user, _ := request.UserFrom(r.Context())
-	if !conf.Server.EnableCoverArtUpload && !user.IsAdmin {
-		http.Error(w, "cover art upload is disabled", http.StatusForbidden)
+	if !conf.Server.EnableArtworkUpload && !user.IsAdmin {
+		http.Error(w, "artwork upload is disabled", http.StatusForbidden)
 		return false
 	}
 	return true