fix: keep sse streams alive behind proxies

server.mjs

@@ -328,10 +328,24 @@ app.get('/events', (req, res) => {
     'Cache-Control': 'no-cache',
     'Connection': 'keep-alive',
     'Access-Control-Allow-Origin': '*',
+    'X-Accel-Buffering': 'no',
   });
+  res.write('retry: 10000\n');
   res.write('data: {"type":"connected"}\n\n');
+  const heartbeatMs = Math.max(5000, config.sseHeartbeatIntervalMs || 25000);
+  const heartbeat = setInterval(() => {
+    try {
+      res.write(`: heartbeat ${new Date().toISOString()}\n\n`);
+    } catch {
+      clearInterval(heartbeat);
+      sseClients.delete(res);
+    }
+  }, heartbeatMs);
   sseClients.add(res);
-  req.on('close', () => sseClients.delete(res));
+  req.on('close', () => {
+    clearInterval(heartbeat);
+    sseClients.delete(res);
+  });
 });
 
 function broadcast(data) {