Audiobook and ebook libraries previously never entered the Recently
Added pipeline: availability detection only ran for TV/movie/mixed
libraries and release_events only knew episode/movie kinds, so server
channels (Discord/generic webhooks) could not announce new audiobooks
or ebooks.
Generalize the movie path into a flat-item-kind registry
(internal/notifications/item_kind.go) driving availability detection,
recording, channel toggles, payload rendering, test fixtures, and the
admin backfill seeder. New kinds share a kind-discriminated
item_availability table; movie_availability stays as-is. Channels gain
notify_new_audiobooks/notify_new_ebooks toggles (default on, additive
API fields) and embeds carry the author from item_people. Flood-safe by
construction: existing libraries seed silently on their first
post-upgrade full scan.
Extract internal/librarykind to replace the is*LibraryType helper
copies that had drifted across scanner, libraryingest, and metadata
(metadata's movie check silently included mixed; now spelled
explicitly).
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
