feat(commerce): implement CouponService with 5 methods + DTOs (#858)
- create(code, type, value, maxUses, expiresAt) → Coupon
- validate(code, order) → ValidationResult
- apply(coupon, order) → Order (mutates line-item totals)
- expire(coupon) → void
- report() → array of redemption stats
Data/Coupon.php and Data/ValidationResult.php as readonly DTOs.
Pest unit tests with _Good/_Bad/_Ugly per AX-10 for all 5 methods.
pint/pest skipped (vendor binaries missing in sandbox).
Legacy helpers in CouponService preserved.
Co-authored-by: Codex <noreply@openai.com>
Closes tasks.lthn.sh/view.php?id=858