The /api/v1/library/tracks endpoint called search_tracks() to get
DatabaseTrack objects, then immediately called api_get_tracks_by_ids()
to re-hydrate full rows for serialization. Two round trips per search.
Added api_search_tracks() that returns dict rows with all track columns
plus artist_name, album_title, and album_thumb_url in a single query.
The basic and fuzzy search helpers were refactored to share raw-row
implementations, so the existing search_tracks() still returns
DatabaseTrack objects for the many internal callers that depend on
that shape (matching pipeline, repair worker, web UI search).
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