Audiobook metadata matching: narrator, series position, and covers for scanned audiobooks

[cubman3134]

#139 reads what's in the tags, and audiobook tagging in the wild is poor — plenty of collections are a folder name and a bitrate. #134's enrichment path resolves books through the AIO Catalog addon (Google Books / Open Library), but audiobooks have their own fields that path doesn't supply, and one of them is the field listeners actually browse by.

What's missing specifically

• Narrator. For audiobooks this is closer to "director" than to a footnote — people choose editions by it, follow narrators across series, and abandon books over them. Nothing in our stack supplies it.
• Series and position — audiobook series numbering is frequently absent from tags and present in the title string (Book 3 of …), so it needs matching rather than parsing.
• Cover, description, publication year, runtime for items whose tags carry none.

Proposed behaviour

• Reuse the enrichment pattern, don't invent one: the Local library: fetch artwork and plot for scanned files, not just an id #73/Local book & comic library: scan, EPUB metadata, covers, and series grouping #134 shape — resolve the item to a catalog id, fetch metadata, persist into MetaCache — applies unchanged. Local tags always win; this fills blanks.
• Sources through the addon model, deliberately. Open Library and Google Books have official public APIs and are the sane defaults. The audiobook-specific data the ecosystem leans on (Audible's catalogue — narrator, series, ASIN) has no official public API; the tools that use it rely on unofficial endpoints. That's exactly the kind of source that belongs behind an addon rather than compiled into the app: an addon declaring metaFor: ["audiobook"] can serve it, the aggregator merges it by role precedence like any other provider, and the app stays neutral about where it came from. Consistent with how the four game-art providers already work, and with the project's posture generally.
• Match confidence, surfaced. Audiobook titles match badly (abridged vs unabridged, dramatizations, re-recordings with different narrators). Show what was matched and let the user reject it — Per-item metadata editor: fix a wrong scrape without re-matching the whole library #24's per-item metadata editor is the natural home for the correction, and a bad audiobook match is more visible than a bad film match because the narrator will simply be wrong.
• Narrator becomes a browse dimension in Local audiobook library: authors, narrators, series, and multi-file books #139's shelf and a filter field in Saved filter presets for the game library #63 — that's the payoff, and it costs little once the field exists. It pairs naturally with Advanced music tags: multi-value fields, composers, and cue sheets #196's composer work: both are "the second name that matters" for their medium.

Not in scope: writing matched metadata back into the files (tagger's job — same call as #152 and the Calibre sweep), and matching for server-owned items, where the server supplies its own metadata (#197 for Audiobookshelf, EverythingBoxServer#24 for ours).