Pre-apply Stripe coupons to product prices

[simonhamp]

What & why

Lets admins offer a discount on a product (e.g. subscriber pricing for the Masterclass) by attaching an existing Stripe coupon to a product price — no need to create a separate Stripe price. When a buyer's resolved price carries a coupon, it is pre-applied to the Stripe checkout session so the discount happens automatically.

This started from wanting to give subscribers ~$100 off the Masterclass without duplicating prices in Stripe.

Changes

• Schema: nullable stripe_coupon_id column on product_prices.
• Checkout (CartController + course.checkout route): when the resolved price has a coupon, the session is created with discounts => [['coupon' => ...]]. The full price is charged as the line item and Stripe deducts the discount itself. Because Stripe rejects allow_promotion_codes alongside discounts, the manual promo-code box is dropped on those sessions only. Multiple coupons in one cart → first applied, rest logged.
• Display: the course pages (/course and the /dashboard/course Livewire component) now show the real price from the database per user. The discounted amount is derived from the coupon's actual Stripe configuration (amount_off / percent_off), fetched and cached for 10 minutes, with a safe fallback to full price if the coupon can't be resolved. This replaces the previously hardcoded 199/299 values.
• Price resolution: Product::getBestPriceForUser() now tie-breaks equal amounts by tier priority, so a coupon-bearing subscriber price wins over an equal-priced regular row.
• Admin: the ProductResource pricing form and table expose the coupon field, with helper text explaining that the amount stays at full price.

Admin usage

1. Create a coupon in the Stripe dashboard and copy its coupon ID.
2. On the product's pricing, add a price row for the relevant tier at full price and paste the coupon ID.
3. Buyers on that price see the discounted amount on-site and get the coupon pre-applied at checkout.

Notes / caveats

• A pre-applied coupon means buyers can't also type a promotion code on that checkout (Stripe allows one or the other).
• If a coupon is deleted/expired in Stripe, clear the field in the admin at the same time — otherwise a checkout attempt within the 10-minute cache window would error.

Testing

• New CheckoutCouponTest (9 tests): tier resolution, amount_off / percent_off display math, coupon-fetch-failure fallback, and both checkout flows with/without a coupon.
• Updated CoursePageTest and CourseContentTest to assert DB-driven and subscriber-discounted pricing.
• All affected suites pass; pint clean.

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