Fix CI type-check failures: remove merger import, add missing exports

- Remove import of ../matchers/merger.js (from a different PR branch)
  and all code that depended on it (mergeAndUpsertBook, remergeBook,
  getMultiSourceBookIds, source blob parsing helpers)
- Add getAllBooks() export needed by pipeline.ts classify stage
- Add setBookSubgenresWithMeta() export that writes confidence and
  source metadata to the book_subgenres table