Residual per-track wishlist downloads (single tracks from different
albums, below the album-bundle threshold) were producing folders
without a year subfolder whenever the wishlist row carried a stale
``track_number=1`` from an older payload default.
Why: ``core/downloads/candidates.py`` had a single API-fetch branch
that served two concerns — resolving the track position AND
hydrating the lean ``spotify_album_context`` (release_date /
total_tracks / cover image) — gated entirely on track_number being
unresolved. When the wishlist row's ``track_number`` happened to
be 1 (a poisoned default rather than a real value), the gate
short-circuited and the album hydration the same call would have
done was skipped. Deezer-sourced discovery matches don't ship
release_date in their search-result album shape, so without the
backfill the folder lost its year.
The two concerns split:
- track_number resolution keeps its track_info → track object →
API precedence chain. track_info defaults still win.
- album hydration runs whenever release_date or total_tracks are
missing, independent of where (or whether) track_number was
resolved.
The single API round-trip still serves both — the cost contract
is preserved. The side-effect coupling is gone.
Lifted into ``core/downloads/track_metadata_backfill.py``
(``hydrate_download_metadata``) so the precedence chain is pinned
in isolation. 24 unit tests cover the precedence chain, the
poisoned-tn=1 regression case, defensive non-dict/None inputs,
the cost guard (API called at most once per invocation), and
disc_number resolution.
Also lands the upstream piece: ``core/wishlist/routes.py:_build_track_data``
no longer defaults ``track_number=1`` / ``disc_number=1`` /
``total_tracks=1`` / ``release_date=''`` when the library-modal add
payload omits them. Missing values now flow through as ``None`` so
the downstream pipeline can detect-and-recover instead of locking
to a fake position.
