Cart::checkoutSession() now inspects each line's price type and selects the
session mode automatically: `subscription` when the cart carries any recurring
price, `payment` otherwise. Recurring lines reuse a synced Stripe Price
(stripe_price_id) when present and fall back to dynamic price_data with a
`recurring` block otherwise; quarterly cadence maps to a 3-month interval since
Stripe has no native quarter. The cart id is propagated via subscription_data
metadata for webhook mapping.
Mixing recurring and one-time prices in one cart throws the new
MixedCheckoutModeException, since a Stripe Checkout session is single-mode.
The recurring resolver tolerates both the package's enum-cast price model and a
host model storing type/interval as plain strings, so it keeps working when
shop.models.product_price is overridden.
