feat(library): remove tracks from the library without touching the file

"Remove from library" deletes the audio_files row and records the path
as excluded, so the next scan does not import it again. Without the
exclusion the operation undoes itself on the next scan, which is worse
than not having it at all; the file on disk is never touched, which is
the promise the confirmation copy will make.

The soft scan compares the number of audio files on disk against the
number of rows, so both walks now skip excluded paths — otherwise the
two counts disagree forever and every launch queues a full scan of the
whole library. A full rescan clears the exclusions, which is the only
way back for a path removed by mistake until there is a UI for it.

backend/database/sql/queries/audio_files.sql

@@ -343,3 +343,7 @@ JOIN audio_files af ON af.recording_id = r.id
 WHERE r.mbid IN (sqlc.slice('mbids'))
   AND af.library_id = ?
 ORDER BY af.file_path;
+
+-- name: GetAudioFilesByPaths :many
+SELECT id, library_id, file_path, group_key FROM audio_files
+WHERE file_path IN (sqlc.slice('paths'));