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/playlist_tracks.sql

@@ -1,10 +1,16 @@
 CREATE TABLE IF NOT EXISTS playlist_tracks (
     id INTEGER PRIMARY KEY,
     playlist_id INTEGER NOT NULL,
-    audio_file_id INTEGER NOT NULL,
+    audio_file_id INTEGER,
     position INTEGER NOT NULL,
+    phantom_title TEXT,
+    phantom_artist TEXT,
+    phantom_album TEXT,
+    phantom_duration_ms INTEGER,
+    phantom_genre TEXT,
+    phantom_cover_art_path TEXT,
     FOREIGN KEY(playlist_id) REFERENCES playlists(id) ON DELETE CASCADE,
-    FOREIGN KEY(audio_file_id) REFERENCES audio_files(id) ON DELETE CASCADE
+    FOREIGN KEY(audio_file_id) REFERENCES audio_files(id) ON DELETE SET NULL
 );
 
 CREATE INDEX IF NOT EXISTS idx_playlist_tracks_playlist_id