Read Audiobookshelf's metadata.json as a metadata layer

[KodeStar]

When AudioSilo is pointed at a library Audiobookshelf manages, each book folder may hold the metadata.json ABS keeps ("Store metadata with item"). It often carries the ASIN/ISBN the tags lack, plus the corrections and matches made in ABS. This reads it.

Precedence: path → tags → ABS file → community match → admin edit, as ABS itself ranks the file above the tags. Path-first libraries included.

Fields read:

• title (not subtitle)
• authors and narrators, joined with ", "
• series "Name #seq": the first is the main series; the rest go to more_series
• publishedDate, else publishedYear
• description (HTML to plain text)
• asin, isbn

Genres, tags, publisher, language and chapters are not read. An empty value never clears a tag's. Each value goes through the field's edit rules, so an ASIN from the file makes the book matched: Health, /meta, the community chapter check and refresh runs all work.

Scope: folder books only. A loose file at the root never takes a metadata.json beside it. An ignore rule metadata.json turns the feature off for a library.

Storage and scans:

• Migration 0046 adds books.abs_sig, abs_indexed_at and abs_file: derived from the file, so rebuildable.
• Discovery records each folder's file signature during the walk.
• An unchanged book whose file is new, changed or gone is re-read with no re-index (SetABSFiles, batched beside the path backfill).
• After upgrade, only books that have a file are touched.
• A read failure keeps the last reading.
• A book an older server re-indexed is re-resolved.
• Links are not followed.

Console: a new abs provenance source, shown as an amber "Audiobookshelf" marker, in the legend, in 6 locales, and in STYLEGUIDE tokens.

Tests: parser, layer and precedence, the series position rule, file books, the backfill (including the stale-stamp case), and a scanner lifecycle test (read, edit picked up without re-index, upgrade backfill, ignore rule, removal, IndexPath, unreadable-file retry).

Checked on a real server with the user-posted sample file: the book page shows every value marked Audiobookshelf, in both themes.

Docs: KodeStar/audiosilo-docs#134. Not a wire change.

Known gaps (not fixed here):

• A position read from an ABS title shows as tag.
• The Revert tooltip says "File tag:" for ABS values, as it already does for path values.
• Authors are joined with ", " like community matches.