Create a valid EU e-invoice (UBL 2.1) from simple JSON

Turns JSON (seller, buyer, lines with VAT category/rate, payment) into a UBL 2.1 invoice or credit note in format en16931 (plain EN 16931), xrechnung (German XRechnung 3.0) or peppol-bis3 (Peppol BIS Billing 3.0 structure; Peppol-specific rules not checked yet). Totals and the VAT breakdown are computed for you; the result is validated before it is returned (valid, errors, xml). Optional: tax_point_date (BT-7), party company_id + company_id_scheme (BT-30/BT-47), seller legal_form (BT-33). Example body: GET /einvoice/sample. Free tier shared, then x402 $0.08.