- The change stream sends the first notice at once and collapses the
notices of the next 0.4 s into one trailing change, so a busy scan costs
an open page one re-read per 0.4 s at most (COALESCE_SECONDS).
- Queue items answer thumbnail_url, the same URL the Images listing builds
(one shared helper, media_thumbnail_url), or null without a thumbnail.
- The "duplicate and refused decision send nothing" test spies on
notify_review_change instead of reading the next event.