fix(metadata): keep a locked language through Hardcover edition hydration (#2757)

[tunglambk]

Summary

deriveAudiobookMetadataFromEdition only checked book.Language == "" before filling the language from the chosen Hardcover audio edition. A manual edit that clears the language locks the field (internal/api/books.go:483), and locked has to mean locked even when the value the user saved is empty — clearing a wrong language is a decision, not a gap for the provider to fill. Hydration wrote the edition's language straight over the lock, and HydrateHardcoverEditions persisted it because BookUpdater.Update writes language unconditionally.

The language branch now also tests book.IsFieldLocked(models.BookFieldLanguage), the same guard internal/calibre/importer.go:971 and internal/abs/import_upserts.go:1046 already use. That is the whole fix. Language is the only lockable field this package writes: MediaType, ImageURL and the ASIN promotion in the same function are not in models.LockableBookFields, so there is no matching omission for them and I left them alone. internal/bookhydrate/ has one non-test source file, so this branch is the whole extent of the mistake there.

Closes #2757.

Checklist

• Commits signed off with git commit -s
• Tests added or updated
• docs/DEPLOYMENT.md updated if env vars, config, or upgrade path changed
• Added a changelog fragment under changelog.d/
• Wiki pages updated if user-facing behaviour changed

No wiki change: docs/Metadata-Editing-Wiki.md already says an edited field is kept instead of overwritten by every automatic refresh, and that promise is what this restores. Nothing on the page became inaccurate.

Test plan

The reproduction is a table in internal/bookhydrate/hardcover_locked_language_test.go, using the package's existing newHydrateBook fixture and a stubbed edition fetcher. With the fix reverted, only the locked-empty case fails:

--- FAIL: TestHydrateHardcoverEditionsRespectsLanguageLock (0.36s)
    --- FAIL: TestHydrateHardcoverEditionsRespectsLanguageLock/locked_empty_survives (0.09s)
    hardcover_locked_language_test.go:68: language after hydration = "ger", want ""
    hardcover_locked_language_test.go:75: persisted language = "ger", want ""
    --- PASS: TestHydrateHardcoverEditionsRespectsLanguageLock/locked_non-empty_survives (0.09s)
    --- PASS: TestHydrateHardcoverEditionsRespectsLanguageLock/unlocked_empty_is_filled (0.09s)
    --- PASS: TestHydrateHardcoverEditionsRespectsLanguageLock/unlocked_non-empty_is_kept (0.10s)

The other three cases pass before and after, which is the point of them: a locked non-empty value survives, an unlocked empty language is still filled from the edition, and an unlocked non-empty value is left as it is. Every case promotes the edition's ASIN, so a pass cannot come from hydration not running at all.

With the fix in place:

• gofmt -l . — clean
• go build ./... — exit 0
• go vet ./internal/bookhydrate/ ./internal/api/ — exit 0
• go test -count=1 ./internal/bookhydrate/ — ok
• go test -count=1 ./cmd/... ./internal/... — 12 failures: 2 in internal/api, 1 in internal/db, 9 in internal/importer. The same 12 fail on unmodified main in this sandbox (hardlink and rename on a filesystem that does not support them); I diffed the two failure lists and they are identical.

golangci-lint cannot run in this environment: the available binary is v1.64.8 and the repo config is v2, so it exits with you are using a configuration file for golangci-lint v2 with golangci-lint v1. CI pins v2.11.4, which I could not reproduce here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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