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

.env.example

@@ -30,6 +30,12 @@ MUSIC_SERVICE=SquidWTF
 # Path where downloaded songs will be stored on the host (only applies if STORAGE_MODE=Permanent)
 DOWNLOAD_PATH=./downloads
 
+# Path where favorited external tracks are permanently kept
+KEPT_PATH=./kept
+
+# Path for cache files (Spotify missing tracks, etc.)
+CACHE_PATH=./cache
+
 # ===== SQUIDWTF CONFIGURATION =====
 # Different quality options for SquidWTF. Only FLAC supported right now
 SQUIDWTF_QUALITY=FLAC