Quite some work done here.

assets/game1.js

@@ -69,6 +69,7 @@ document.addEventListener('DOMContentLoaded', async () => {
   const topic = cfgEl.dataset.topic;
   const apiPingUrl = cfgEl.dataset.apiPingUrl;
   const apiEchoUrl = cfgEl.dataset.apiEchoUrl;
+  const userId = cfgEl.dataset.userId;
 
   if (mercurePublicUrl && topic) {
     subscribeToMercure(mercurePublicUrl, topic);
@@ -88,7 +89,7 @@ document.addEventListener('DOMContentLoaded', async () => {
     if (apiEchoUrl) {
       const echo = await fetchJson(apiEchoUrl, {
         method: 'POST',
-        body: { hello: 'from game1.js', ts: new Date().toISOString() },
+        body: { message: 'from game1.js', ts: new Date().toISOString(), user: userId },
       });
       console.log('[API][game1] echo →', echo);
     } else {