Connect callback receiver

A callback receiver starter for inspecting synthetic claim delivery before Test Integration. This starter is intentionally dependency-light: inspect it, adapt the shape locally, then replace placeholders only after Test Integration credentials are issued from the authenticated CAIRL workspace.

Starter code

// SANDBOX_SYNTHETIC=true
app.get("/api/cairl/callback", (req, res) => {
  res.json({
    received: req.query,
    synthetic: true,
    billable: false,
  });
});

No hidden internal credentials.

Synthetic-only preview state.

Graduate through Test Integration before live.