Create thumbnail of proof images

[raphael0202]

Following https://github.com/openfoodfacts/smooth-app/issues/5326#issuecomment-2185995548, create 400px version of all proof images. We can use the same format as on Product Opener, {IMAGEID}.400.{EXT}

And also compress/convert in .webp ? see https://github.com/openfoodfacts/open-prices/issues/487

[monsieurtanuki]

@raphael0202 @raphodn ping

[raphodn]

@raphael0202 @raphodn ping

Once #355 (Django) is deployed in prod we'll put it on the todo

[monsieurtanuki]