Libraries managed by Audiobookshelf keep a metadata.json next to each
book (title, authors, series+sequence, genres/tags, publisher,
description, isbn/asin, language, published year/date) - and no
metadata.opf. The scanner silently ignored those files: deleting them
changed nothing, and their data never reached the database.
Parse them as a first-class sidecar in extractMetadata, priority
metadata.opf -> metadata.json -> embedded media. Only fields with a
matching media_items column are mapped; narrators, subtitle, explicit,
abridged, and chapters are deliberately skipped.
Cover handling is unchanged: the existing findSidecarCover priority
(cover.jpg / folder.jpg / {basename}.jpg) applies to the sidecar branch
exactly as it does for Calibre.