`merge` decided by whether a usable still existed, so the same clip was included or
dropped depending on whether ffmpeg happened to extract a poster — a video WITH a
thumbnail was queued and shown as a frozen frame, one without was skipped. Keyed on
the mime type instead: the projector shows stills only.
The test factory casts through `as unknown as FeedUpload`, so adding a field the queue
reads does not fail typechecking — it fails at runtime, which is how this surfaced as
ten broken tests rather than a compile error. The factory now carries `mime_type` and
the comment says why keeping it in step matters.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
89ca819529