Stripe integration — product, checkout flow, webhooks

[TheTrueAI]

Phase: 2 — Monetization

Integrate Stripe for paid newsletter subscriptions (~€5-9/month):

• Create Stripe product and price
• Add Stripe Checkout flow — redirect to payment after DOI confirmation
• Add stripe_customer_id, stripe_subscription_id, payment_status columns to subscribers table
• Implement Stripe webhook handler for invoice.paid, customer.subscription.deleted, invoice.payment_failed
• Handle payment failures gracefully (grace period? immediate deactivation?)

See ROADMAP.md Phase 2.1 and 2.2 for the free vs paid tier breakdown.