FluentCart coupons and discounts

Extend the calculation pipeline instead of altering displayed totals. Revalidate every coupon against current server-owned cart data.

Read coupon-pipeline.md before implementing a virtual coupon, new rule, custom type, or recurring discount.

Use the canonical model

Coupon stores code, type, amount, priority, status, stackable, show_on_checkout, dates, use_count, and JSON conditions. AppliedCoupon stores the order-time snapshot. Never reconstruct historical discounts from the currently editable Coupon.

Use CouponResource for admin CRUD because it validates and converts fixed amounts/limits. In 1.6.0: