increase mercure heartbeat

docker/mercure/caddy/Caddyfile

@@ -21,7 +21,7 @@ route {
         publish_origins https://local.ap2i.opentalent.fr http://ap2i https://local.mercure.opentalent.fr
         demo   # remove in prod
         subscriptions
-        heartbeat 0s
+        heartbeat 40s
     }
 
     respond /healthz 200