Create a new Docs module that consolidates PDF and eBook handling into a
unified system with subjects and authors support. This replaces the
separate PDF/eBook modelers with a single Doc model backed by an
alembic migration (rename ebook to doc).
Key changes:
- New modules/docs package with models, API, extractors, and lib
- Subject and author metadata extraction with case-insensitive
normalization for cleaner PDF metadata
- Frontend Docs pages with AuthorsPage, SubjectsPage using
CollectionTable, and doc card deletion
- Global search integration for subjects and authors
- Generic collection item count support
- Frontend file card metadata adjusted to match collection sort
- Bump yt-dlp version
