feat(10-01): update SQL schema files for multi-library fresh installs

- Create _libraries.sql with libraries table (name, path, created_at)
- Add library_id FK column and index to audio_files.sql
- Update playlist_tracks.sql with nullable audio_file_id, SET NULL FK, and 6 phantom columns
- Add af.library_id to track_metadata VIEW
- Regenerate sqlc code for updated schemas
- Fix playlist.go to use sql.NullInt64 for nullable audio_file_id

backend/database/sql/schemas/track_metadata_view.sql

@@ -22,7 +22,8 @@ SELECT
     af.bit_depth,
     af.channels,
     af.bitrate,
-    af.file_size
+    af.file_size,
+    af.library_id
 FROM audio_files af
 LEFT JOIN recordings r ON af.recording_id = r.id
 LEFT JOIN artist_credit ac ON r.artist_credit_id = ac.id