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>

backend/main.go

@@ -125,6 +125,11 @@ func main() {
 	r.GET("/spotify/recent", store.RecentlyPlayed)
 	// r.POST("/spotify", store.SendSong)
 
+	// RADIO
+	admin.POST("/radio/upload", store.UploadRadioSong)
+	admin.GET("/radio/songs", store.ListRadioSongs)
+	admin.DELETE("/radio/songs/:filename", store.DeleteRadioSong)
+
 	// MESSAGES
 	r.GET("/ws", store.ConnectWebSocket)
 	protected.POST("/messages/upload", store.UploadMessageFile)