- feed_columns() is the single feed-row projection, consumed by both /feed and the
playlist-detail query (was copy-pasted, drift risk on every new column).
- _replace_items_from_youtube() dedups the YouTube playlist-mirror block shared by
sync_user_playlists and repull_playlist.
- _next_playlist_position/_next_item_position/_add_item replace the append-position
idiom (x4) and the exists-then-insert blocks (x2).