fix(13-02): auto-resolve phantom playlist tracks after library scan

- Add phantom_file_path column to playlist_tracks (migration 7)
- Store original file_path during RemoveLibrary phantom metadata population
- After each successful scan, UPDATE phantom tracks whose phantom_file_path
  now matches an audio_files row, re-linking them and clearing phantom metadata
- Update schema file, sqlc generated code, and database test for new column

backend/database/sql/sqlcgen/models.go

@@ -90,6 +90,7 @@ type PlaylistTrack struct {
 	PhantomDurationMs   sql.NullInt64
 	PhantomGenre        sql.NullString
 	PhantomCoverArtPath sql.NullString
+	PhantomFilePath     sql.NullString
 }
 
 type Queue struct {

backend/database/sql/sqlcgen/playlists.sql.go

@@ -16,7 +16,7 @@ INSERT INTO playlist_tracks (
     phantom_title, phantom_artist, phantom_album,
     phantom_duration_ms, phantom_genre, phantom_cover_art_path
 ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
-RETURNING id, playlist_id, audio_file_id, position, phantom_title, phantom_artist, phantom_album, phantom_duration_ms, phantom_genre, phantom_cover_art_path
+RETURNING id, playlist_id, audio_file_id, position, phantom_title, phantom_artist, phantom_album, phantom_duration_ms, phantom_genre, phantom_cover_art_path, phantom_file_path
 `
 
 type AddPlaylistTrackParams struct {
@@ -55,6 +55,7 @@ func (q *Queries) AddPlaylistTrack(ctx context.Context, arg AddPlaylistTrackPara
 		&i.PhantomDurationMs,
 		&i.PhantomGenre,
 		&i.PhantomCoverArtPath,
+		&i.PhantomFilePath,
 	)
 	return i, err
 }