Import album search silently fell through to the next source in
METADATA_SOURCE_PRIORITY when the configured primary returned zero
matches — intentional behavior shared with the auto-import worker
(see core/auto_import_worker.py:1316). With MusicBrainz selected and
a query MB couldn't resolve, users saw Deezer cards with no indication
their primary was bypassed.
Backend now echoes `primary_source` on /api/import/search/albums,
/api/import/search/tracks, and /api/import/staging/suggestions.
Frontend renders a per-card 'via {source}' badge when the served
source differs from the primary, plus a banner above the grid when
every card came from a fallback source. Fallback semantics unchanged.
Also collapses an inline duplicate of _renderSuggestionCard inside
importPageSearchAlbum into a single shared renderer.
Regression test pins the contract: response carries primary_source +
per-album source when the chain falls back.
eba7f61e04