Three-endpoint, per-(user,profile)-scoped bookmark surface for the
Audiobookshelf-compatibility API. Mirrors the canonical continuum
plugin's "key by (item, time)" model, with silo's profile-isolation
overlay and the existing nil-safe socket publish wrapper for
realtime user_updated events. One migration (148_abs_bookmarks),
one new store, one new handlers file. Ready for the implementation
plan once the user signs off.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
