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 {