Approved design for the second sub-project of Phase 1: manual user
collections (named groupings of audiobooks) and ordered playlists
(queue with cover image). Both profile-scoped, with cross-user
public visibility. Includes migrations 149-152, two REST surfaces
sharing a uniform CRUD shape, in-memory test fakes parallel to the
bookmark sub-project's harness, and the same anti-enumeration 404
security pattern.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
