[FR - Grab Posters] Toggle to Only Grab Locked Posters

[tronyx]

Per Discord discussion, I think it would be great if you could add a toggle to the grab_all_posters.py script to only grab locked posters, IE: only the ones that the Plex Server owner changed, rather than all of them.

[chazlarson]

I've got an implementation of this, but it doesn't appear that changing the art in the UI locks the field. Setting it with Kometa does, but not the UI.