Adding metadata sources

Follow the repository’s canonical provider guide. It is the maintained checklist for API suitability, terms, attribution, privacy, implementation, and real-response fixtures.

The boundary is MetadataProviderPort, in src/application/ports/metadata-provider.ts. Providers live under src/adapters/metadata-providers/ and use its existing registry.

Reuse the shared HTTP/error helpers and test against recorded service responses.

A source supplies metadata, never manga pages, downloads, or reader links. If an API cannot provide the required volume data or does not meet the acceptance rules, discuss it before implementing an adapter.