feat(10-02): add sqlc queries for libraries and update playlist queries for phantom support

- Create libraries.sql with 7 CRUD queries (create, get, get-by-path, list, update, delete, count)
- Update playlists.sql: AddPlaylistTrack now accepts 9 params including phantom metadata
- Update playlist queries to use LEFT JOIN for nullable audio_file_id
- Add GetTrackPhantomMetadata helper query for eager phantom population
- Add is_phantom computed column to metadata queries
- Add GetAudioFilesByLibrary and CountAudioFilesByLibrary queries
- Regenerate all sqlc code

backend/database/sql/queries/audio_files.sql

@@ -127,6 +127,12 @@ SELECT id, file_path, title, artist_name
 FROM track_metadata
 WHERE file_path IN (sqlc.slice('paths'));
 
+-- name: GetAudioFilesByLibrary :many
+SELECT * FROM audio_files WHERE library_id = ?;
+
+-- name: CountAudioFilesByLibrary :one
+SELECT COUNT(*) AS count FROM audio_files WHERE library_id = ?;
+
 -- name: DeleteAllAudioFiles :exec
 DELETE FROM audio_files;