A page switch reads the next page while the canvas still shows the page
being left. The media check paired the next page's assets with the
canvas's images, so every picture of the page left looked missing from
the server and was uploaded again from its file on disk, while the next
page's own images were never checked.

Take the images from the page snapshot itself, and check a page again
whenever the page changes, not only when its assets do.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>