Serves the same PDF the result email attaches.

Do not construct this URL. Take report_url from GET /skin/delivery/{batch_id}
and use it as given: the signature it carries is the only credential, and no
bearer token is accepted here. Hand the URL straight to the messaging provider,
which fetches the document itself.

The link is valid for seven days by default. The expiry is covered by the
signature, so editing expires invalidates it. Request a fresh payload rather
than storing the link.