Add Stripe Checkout to my {{tech_stack}} project so customers can pay for {{product_or_service}} from {{page_or_feature}}. Read the existing code and show me a short plan first. Requirements: - Create the Checkout Session on the server with price IDs from server config; never trust an amount from the browser - One-time payments, plus subscriptions if the project has plans - Fulfill orders only from the webhook, never the success page: verify the signature, handle completed, delayed and failed payments, and ignore duplicate events - An orders table with status, amount, currency, Stripe IDs and customer, protected by access rules - A success page that checks the session on the server, and a cancel page that keeps the cart - Subscriptions get the customer portal, plus handling for failed renewals and cancellations - Disable the pay button while the session is created, to prevent double charges - Secret key and webhook secret only in server environment variables; only the publishable key reaches the browser Work in test mode, and give me steps to forward webhooks locally with the Stripe CLI and test a success, a decline and a card that needs authentication. When you're done, list the files changed, environment variables to set, your assumptions, and a go-live checklist.