Move image refresh logic from PeopleValidator (which runs during library
scans) into PeopleValidationTask (the "Refresh People" scheduled task).
This keeps library scans fast while ensuring the scheduled task fetches
missing images from remote providers like TMDB.
People missing a Primary image or overview get refreshed with
MetadataRefreshMode.Default instead of ValidationOnly, with a 30-day
cooldown to avoid hammering providers for people they have no data for.
Fixes jellyfin#8103
dc92e3b0e4