fix: stabilize color mapping by using persistent global hash and add SSE heartbeat

backend/index.js

@@ -74,9 +74,15 @@ app.get('/api/stream', (req, res) => {
     // Send initial connection event
     res.write('data: {"connected": true}\n\n');
 
+    // Keep connection alive through Nginx/Cloudflare
+    const keepAlive = setInterval(() => {
+        res.write(':\n\n'); // SSE comment to prevent idle timeout
+    }, 15000);
+
     sseClients.add(res);
 
     req.on('close', () => {
+        clearInterval(keepAlive);
         sseClients.delete(res);
     });
 });