Public Stripe Checkout redirect target (browser hand-off)

Stripe redirects the user's browser here after checkout. This endpoint is intentionally unauthenticated — a browser redirect carries no bearer token, and gating it returned "No token provided" (UNAUTHORIZED) after a successful charge. Subscription activation is webhook-driven; this performs a best-effort idempotent fulfillment and then hands the browser back to the desktop app via an openhuman://payment/success deep link.