When customers pay for ADP quotations through the portal, the system
was charging the full order amount (ADP + client portions combined).
Now correctly charges only the client portion (25% for REG clients).
Changes:
- Override _get_prepayment_required_amount() to return client portion
- Override _has_to_be_paid() to skip payment for 100% ADP-funded orders
- Add portal controller to cap payment amount at client portion
- Add portal template showing ADP funding breakdown to customer
