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>

nginx/nginx.conf.template

@@ -9,7 +9,7 @@ http {
     server_tokens off;
     charset utf-8;
 
-    client_max_body_size 10M;
+    client_max_body_size 50M;
 
     limit_req_zone $binary_remote_addr zone=login:10m rate=5r/m;
     limit_req_zone $binary_remote_addr zone=api:10m rate=30r/s;