Add reply indicator to top posts showing "Reply to @handle"
Posts that are replies now display a small heading linking to the parent
post on bsky.app. Reply parent URI and author handle are stored in
ClickHouse at ingest time — the backfill path gets handles from the
AppView feed response, and the Jetstream path looks up handles from
the tracked-users map (covers replies to other tracked users).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>