Track Redownload had been doing parallel multi-source metadata search
across every configured source the whole time; Enhance Quality was
running a single-source primary fallback that returned junk matches
with empty fields when the primary was iTunes (Discord report:
"unknown artist - unknown album - unknown track" wishlist entries
for users with neither Spotify nor Deezer connected).
Lift the redownload search into core/metadata/multi_source_search.py
and point both flows at it. Same scoring, same per-source query
optimization (Deezer's structured artist:/track: form), same
current-match flagging via stored source IDs.
ArtistQualityDeps now takes get_metadata_search_sources (returns
[(name, client), ...] for every configured source) instead of the
single-primary get_metadata_fallback_client + get_metadata_fallback_source.
Spotify direct-lookup stays as a fast-path optimization (only Spotify
exposes get_track_details(id) returning rich raw payload); when it
doesn't fire, the multi-source parallel search picks the cross-source
best match. Empty-field matches still rejected before wishlist add.
Tests: _build_deps helper updated to accept the new search_sources
contract while preserving fallback_client/fallback_source ergonomics.
Reframed tests for the new semantics — direct-lookup is no longer
gated on Spotify being the active primary; failure reason now lists
every searched source. Added a test pinning the no-sources-configured
prompt. 17/17 quality tests green, 2128/2128 full suite green.
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