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.

conf/configuration.go

@@ -78,7 +78,7 @@ type configOptions struct {
 	EnableFavourites                bool
 	EnableStarRating                bool
 	EnableUserEditing               bool
-	EnableCoverArtUpload            bool
+	EnableArtworkUpload             bool
 	EnableSharing                   bool
 	ShareURL                        string
 	DefaultShareExpiration          time.Duration
@@ -689,7 +689,7 @@ func setViperDefaults() {
 	viper.SetDefault("enablereplaygain", true)
 	viper.SetDefault("enablecoveranimation", true)
 	viper.SetDefault("enablenowplaying", true)
-	viper.SetDefault("enablecoverartupload", true)
+	viper.SetDefault("enableartworkupload", true)
 	viper.SetDefault("enablesharing", false)
 	viper.SetDefault("shareurl", "")
 	viper.SetDefault("defaultshareexpiration", 8760*time.Hour)