Fixed: videos not loading on projects reopened after a week

The cloud storage bucket uses presigned URLs that expire after 7 days. Those URLs were cached in IndexedDB, so when you reopened a project after a week, the browser got a 403 and reported "MEDIA_ELEMENT_ERROR: Format error" — misleading if you didn't know about signed URLs.

Now, whenever a project loads, the client calls the server to regenerate fresh URLs for every asset and swaps them into the media store. Old projects start playing again immediately.