Add Stripe subscriptions and a billing page

Add Stripe subscription billing to my [FRAMEWORK] app so users can pay for a paid plan.

Plans:
- Free: [WHAT THEY GET]
- Pro at [PRICE] per month: [WHAT THEY GET]

Build all of this:
1. A pricing section with a subscribe button that opens Stripe Checkout in subscription mode.
2. A webhook endpoint that listens for checkout.session.completed, customer.subscription.updated, and customer.subscription.deleted, and updates the user's plan and status in my database.
3. A billing page where a logged-in user can see their current plan and open the Stripe Customer Portal to update or cancel.
4. A single helper I can call anywhere to check if the current user is on the Pro plan, so I can gate features.

Rules:
- Never trust the client for payment state. The database is only ever updated from a verified webhook, and you must verify the Stripe webhook signature.
- Use test-mode keys. Tell me every environment variable I need and where to find it in the Stripe dashboard.
- Store the stripe_customer_id and subscription status on my user record.

Before coding, list the files you will create or change and the exact Stripe products and prices I must create first. Then implement it, and finish with steps to test the full flow with a Stripe test card.