Adds the pgx-backed PlaylistStore impl, wires it into BuildABSHandler
when a Pool is present, and registers all ten playlists routes under
both /abs/api and /api prefixes inside the existing bearerAuth group.
AddPlaylistItem computes position = MAX+1 inside the INSERT (one
round-trip, no read-before-write race); both add and remove run in
transactions so the parent's updated_at bump is atomic.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
