Second leak of the same class: redownload_start built full album_data (release_date/album_type/
total_tracks) only in the Spotify branch. The iTunes and Deezer branches set just track/disc number
and left album_data lean ({'name': ...}), so single-track redownloads on those sources dropped the
$year — same symptom as #915 in the add/download path.
Fix: both branches now fetch the album via get_album_for_source (cached, source-aware) and build
album_data through the shared _album_data_from_source helper, mirroring the Spotify branch. Falls
back to the lean default if the fetch returns nothing (no regression). get_album is cached on both
iTunes and Deezer, so no extra API cost.
Tests: _album_data_from_source (full build, image-url fallback, defaults). 694 library+downloads
tests green.
