fix: raise JSON body limit to 10mb for cover art upload

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

backend/novasonic-server.js

@@ -31,7 +31,7 @@ app.use((req, res, next) => {
   if (req.method === 'OPTIONS') { res.sendStatus(204); return; }
   next();
 });
-app.use(express.json());
+app.use(express.json({ limit: '10mb' }));
 
 let ytdlpVersion = 'unknown';
 try {