feat: enrich downloaded audio files with extended metadata

- Add metadata enrichment from Spotify, MusicBrainz, and Last.fm
- Introduce metadata.py library for fetching and writing metadata
- Enable toggling enrichment via the UI
- Improve error handling and logging during enrichment
- Update documentation with contributing guidelines

fix: better error handling when all spotiflac services fail to find a…

… song and added title to each page