processUploadedVideo could return success with thumbnailKey: null
when only thumbnail generation failed. The gallery grid
(GridGalleryLayout/JustifiedGalleryLayout) falls back to
`photo.thumbnail_url || photo.url` when there's no thumbnail, so
AuthenticatedImage downloaded the full original video and tried to
render it as an <img> -- a broken tile and a potentially huge
fetch just from opening the gallery.
Falls back to the same ffmpeg-free SVG placeholder the callers
already generate for a total processing failure, so a bare
thumbnail-generation failure degrades to that placeholder too,
never to "no thumbnail at all".
Found by codex review.
Paul Nothaft
69436fba91