Add Stripe Subscriptions

Add subscription billing to this project.

Plans: [LIST YOUR PLANS AND PRICES].

Requirements:
- Create checkout sessions from server code only. The secret key must be a server secret.
- Add a webhook endpoint that verifies the signature before doing anything.
- Treat the webhook, not the browser redirect, as the source of truth for subscription state.
- Store subscription status, plan and current period end against the user.
- Gate features on the stored subscription status.
- Handle cancellation, payment failure and reactivation.
- Make webhook handling idempotent so duplicate deliveries do not double-apply.

Test with a successful card, a declined card, and an abandoned checkout.