Generate the RiskCube invoice for a policy.

Builds the standard RiskCube invoice from the policy record — the same template for every policy and every carrier — and saves it to a NEW storage object recorded in custom_invoice_url. Nothing existing is modified: the carrier's own invoice and invoice_url are left untouched.

When a carrier invoice is on file, its invoice number is reused verbatim and its own total is checked against the policy record; a mismatch comes back as a non-blocking warning so the admin can look into it.