feat(deploy): add portable external-service stack

backend/src/routes/sessions.ts

@@ -77,6 +77,9 @@ export function sessionRoutes(
       "Access-Control-Allow-Origin": origin,
       "Access-Control-Allow-Credentials": "true",
     });
+    // Send the handshake immediately. Without this, Node waits for the first event body and
+    // proxies/clients cannot establish an idle SSE subscription or inspect its headers.
+    reply.raw.flushHeaders();
     const send = (event: string, data: object) => reply.raw.write(`event: ${event}\ndata: ${JSON.stringify(data)}\n\n`);
     const off = d.hub.subscribe(id, send, rt?.bridge.pendingWidget() ?? null);
     req.raw.on("close", off);