[Feature] Payment Integration and Subscription Management

[P4ST4S]

Description:
The pricing page shows three tiers (Free, Pro €29/month, Enterprise) but has no payment integration. CTAs like "Essayer 14 jours gratuits" and "Nous contacter" are non-functional.

Context/Rationale:
Payment integration is essential for SaaS revenue. Without it, you cannot monetize the Pro and Enterprise plans, making the business model non-viable. Users cannot upgrade, and you have no recurring revenue stream.

Location: components/Pricing.tsx:136-144

Implementation Suggestion / To-Do:

1. Integrate Stripe for payments (or LemonSqueezy for EU)
2. Implement webhook handlers for subscription events
3. Create Stripe Customer Portal integration
4. Build plan management UI in user dashboard
5. Add trial period logic (14 days free for Pro)
6. Implement usage-based billing if needed
7. Add invoice generation and receipt emails
8. Handle failed payments and dunning management