The bulk download_discography endpoint picked one metadata client
based on the configured primary source and called .get_album() on
every album with that single client. Albums whose IDs came from a
fallback/provider-specific source (e.g. Deezer-formatted IDs surfaced
through Hydrabase) failed with "Album not found" because the primary
client couldn't resolve them.
Bulk now uses the same source-aware resolver
(core.metadata.album_tracks.get_artist_album_tracks) the working
individual-album endpoint already uses, so the resolver's source-chain
walk finds each album under whichever provider actually has it. Also
adds explicit Discogs and Hydrabase support (the old if/elif chain
silently 500'd for those primaries).
Frontend (library.js + pages-extra.js) now sends a richer
`{ albums: [{id, name, artist_name, source}] }` payload so each album
can be resolved through its own source. The legacy `album_ids` payload
still works as a fallback path.
Closes#399.
