Needle-style checkout readiness

Wire this page to a public Payment Link later.

This is the same lightweight checkout boundary used for the Prompt Operations payment lane: public hosted checkout URLs are safe to place behind pricing CTAs, but no Stripe secrets, webhooks, license keys, or gated downloads belong in the static frontend.

Current state: paid checkout is intentionally not live. Configure PUBLIC_CHECKOUT_URL or PUBLIC_CHECKOUT_URL_PRO only when the signed, hosted final artifact and fulfillment copy are ready.