Add POST /api/v1/stripe/create-subscription-checkout endpoint with
get_or_create_stripe_customer (SELECT FOR UPDATE), plan_id→price_id
mapping from env vars, active subscription guard (409), and
session_id in success URL. Add GET /verify-checkout-session endpoint
for server-side payment verification. Add /subscription-success
frontend page with loading/verified/failed states.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
