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/schemas/playlist_tracks.sql

@@ -9,6 +9,7 @@ CREATE TABLE IF NOT EXISTS playlist_tracks (
     phantom_duration_ms INTEGER,
     phantom_genre TEXT,
     phantom_cover_art_path TEXT,
+    phantom_file_path TEXT,
     FOREIGN KEY(playlist_id) REFERENCES playlists(id) ON DELETE CASCADE,
     FOREIGN KEY(audio_file_id) REFERENCES audio_files(id) ON DELETE SET NULL
 );