feat: extended metadata downloader

[Sudashiii]

No description provided.

[Copilot]

Pull request overview

This PR completes the "extended metadata downloader" feature by adding a new ISBNdb metadata provider, an ApplyMetadataCandidateUseCase that maps user-selected fields from a metadata candidate onto a Book, a strict HTML sanitizer for provider descriptions, and shared author normalization helpers that are now reused across all metadata providers. It also tightens cover-image safety (minimum size + magic-byte signature checks) in ManagedBookCoverService and documents the new environment variables.

Changes:

• New IsbnDbMetadataProvider (factory-gated on ISBNDB_API_KEY) plus shared normalizeAuthor/normalizeAuthorForMatch helpers used by all providers' scoring.
• New ApplyMetadataCandidateUseCase and sanitizeMetadataDescription to safely apply selected candidate fields + cover to a book.
• ManagedBookCoverService now enforces a minimum buffer size and validates image magic bytes for both fetched and embedded covers; env docs updated.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sake/src/lib/utils/author.ts	New shared author normalization helpers.
sake/src/lib/server/infrastructure/metadata-providers/isbndbMetadataProvider.ts	New ISBNdb provider implementation.
sake/src/lib/server/infrastructure/metadata-providers/metadataProviderFactory.ts	Wire ISBNdb into the factory (was Phase 6 stub).
sake/src/lib/server/infrastructure/metadata-providers/{googleBooks,openLibrary,hardcover}MetadataProvider.ts	Use shared normalizeAuthorForMatch for author scoring.
sake/src/lib/server/application/services/MetadataDescriptionSanitizer.ts	New allowlist-based HTML sanitizer for descriptions.
sake/src/lib/server/application/services/ExternalBookMetadataService.ts	Run candidate descriptions through the sanitizer.
sake/src/lib/server/application/use-cases/ApplyMetadataCandidateUseCase.ts	New use case to apply candidate fields + cover to a book.
sake/src/lib/server/application/services/ManagedBookCoverService.ts	Add min-size + magic-byte checks for cover buffers/fetches.
sake/tests/metadata/*, sake/tests/library/managedBookCoverService.test.ts	Unit tests covering all new behavior.
sake/.env.example, sake/.env.docker.selfhosted, README.md	Document new metadata env vars.

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