fix(pricing): require board decision for member special discounts, allow fixed-amount down payment on board offers

- special_discounts gains board_decision_number/board_decision_date; the
  member-facing "special discount" dropdown (fill-form, edit, show, apply)
  now only lists/accepts discounts backed by a board decision instead of
  the full unaudited special_discounts catalog, per client request that the
  dropdown should only read board-approved discounts.
- board_offers gains inst_down_payment_type (percentage/fixed_amount) so a
  board-approved offer's installment down payment (المقدم) can be a fixed
  cash amount, not only a percentage. Wired through BoardOfferService,
  InstallmentCalculator (new min_down_amount override) and
  PaymentLifecycleService so the fixed amount is actually enforced at
  billing time, not just in the admin form.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>