More accurate updated_at field for pictures

This updated_at field of pictures is used for the federated catalog to know what have changed since the last crawl, and this field was only updated in changes on the picture metadata (and on its semantic), but not when we picture was indeed available (after the blurring process).

The updated_at field of pictures is now updated when the picture is prepared (blurred), making it their harvest from the federated catalog more consistent.

Linked to https://forum.geocommuns.fr/t/difference-entre-linstance-ign-et-le-catalogue-complet/3367