Add admin UI for managing radio fallback music

Upload, list, and delete fallback music files from the admin page.
Backend handlers validate file type/size and prevent path traversal.
Nginx max body size increased to 50M to support large audio files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -72,6 +72,7 @@ services:
             - ${OBSIDIAN_DIR}:/backend/notes
             - ./logs:/backend/logs
             - uploads:/backend/uploads
+            - ./icecast2/fallback_music:/backend/fallback_music
 
     db:
         image: postgres:16