Luria Credits Checkout

Buy a Luria credits pack. Lives in main (not luria.py) so it can reuse the billing rails verbatim: payment_orders row + provider pay URL + the same webhooks. plan_code='luria_credits' makes _finalize_order_paid credit the ledger instead of activating a subscription; the pack's credit count rides in discount_meta so finalize never has to re-look-up prices that may have changed between checkout and payment.