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/audio_files.sql

@@ -10,9 +10,14 @@ CREATE TABLE IF NOT EXISTS audio_files (
   bitrate int NOT NULL DEFAULT 0,
   file_size int NOT NULL DEFAULT 0,
   basename text NOT NULL DEFAULT '',
+  library_id int NOT NULL DEFAULT 0,
   FOREIGN KEY(file_type_id) REFERENCES file_types(id),
-  FOREIGN KEY(recording_id) REFERENCES recordings(id)
+  FOREIGN KEY(recording_id) REFERENCES recordings(id),
+  FOREIGN KEY(library_id) REFERENCES libraries(id)
 );
 
 CREATE INDEX IF NOT EXISTS idx_audio_files_recording_id
     ON audio_files(recording_id);
+
+CREATE INDEX IF NOT EXISTS idx_audio_files_library_id
+    ON audio_files(library_id);