feat(metadata): multi-provider abstraction (LoC, DNB, K10plus, Open Library)

[phildenhoff]

Summary

Makes Hardcover one metadata source among several. With no Hardcover key configured, ISBN lookup still works via the keyless library providers.

Live providers: Library of Congress, Deutsche Nationalbibliothek, K10plus (SRU/MARC21), Open Library (JSON), and Hardcover (GraphQL). All five lookups verified end-to-end against the real endpoints.

Backend (src-tauri/src/metadata/)

• Unified BookMetadata model + three provider-agnostic Tauri commands behind enum dispatch (Tauri/specta-safe; provider crosses as a string).
• Config-driven SRU endpoint registry — adding a MARC21 library is a data row, not code (that's how K10plus was added).
• quick-xml MARCXML parser: author inversion ("Last, First" → display order, corporate names left alone), 6xx subjects, 008/041 language, entity decoding (&), and ISBN-match verification.
• Open Library JSON client; Hardcover moved in with unchanged behaviour.
• ISBN 10↔13 equivalence lives here as the single authority.

Frontend

• Capability-driven provider registry + a streaming aggregator that dedupes by ISBN to one whole record (cover backfill is the only borrowed field) and pins the file's ISBN match.
• One unified MetadataSearchModal with source provenance and a designed cover-less placeholder.
• Provider-agnostic hooks for the Edit and Add-Book flows; a "Metadata sources" settings pane; library subjects offered as opt-in tag-suggestion chips.
• Language is parsed but applied later (CDL-2).

Settings migration

Flat hardcoverApiKey/hardcoverAutoLookup → a provider block. Versioned: v0→v1 folds the Hardcover key in; v1→v2 adds K10plus. Keyless libraries default on; existing Hardcover keys preserved.

Also fixes

• Edit Book showing a stale cover after a lookup — cache-bust the single-book cover URL by mtime (list paths stay stat-free at scale).
• Settings panel content shifting sideways when a scrollbar appears.
• Provider-row switch alignment; Add Book modal label truncation.
• "Matches ISBN" picker badge contrast in light and dark.

Testing

• Rust: MARCXML mapping for LoC/DNB/K10plus, ISBN equivalence, entity decoding, cover cache-bust — all green.
• Frontend: aggregator dedup/pin/cover-backfill, settings migration — 166 tests green.
• format:check, bun lint (web + clippy), tsc all clean.
• Verified live: all five providers return correct data; settings migration applied; picker streams merged results with provenance; subject chips add to tags.

Note: British Library was considered but is excluded — its SRU/Z39.50 endpoints remain offline after the 2023 cyberattack. It's a one-row add when they return.

Fixes CDL-7

🤖 Generated with Claude Code

[linear-code]

CDL-7

[github-actions]

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