Add favorite-to-keep feature for external tracks

- When favoriting an external track, automatically copy to /kept folder
- Organized as Artist/Album/Track structure
- Includes cover art if available
- Downloads track first if not already cached
- Add KEPT_PATH and CACHE_PATH volumes to docker-compose
- Update .env.example and README with new feature

docker-compose.yml

@@ -93,6 +93,8 @@ services:
       - Qobuz__Quality=${QOBUZ_QUALITY:-FLAC}
     volumes:
       - ${DOWNLOAD_PATH:-./downloads}:/app/downloads
+      - ${KEPT_PATH:-./kept}:/app/kept
+      - ${CACHE_PATH:-./cache}:/app/cache
 
 networks:
   allstarr-network: