Extends ChangelogEntry with an optional highlights field (editorial,
per-version, plain language -- most versions won't have one) instead
of forking a second content source. One new users column
(lastSeenChangelogVersion, backfilled to current APP_VERSION on
migration so existing users aren't flooded with history), two routes,
one component mirroring NotificationBell's existing bell/badge/dropdown
shape.
Arnaud