Add Stripe Subscriptions to Lovable App

Add Stripe subscription payments to my Lovable app.

REQUIREMENTS:
1. Create a Pricing page with 3 tiers (Free, Pro, Enterprise) displayed as cards with feature comparison, monthly/annual toggle, and a "Subscribe" button per tier.
2. On clicking Subscribe, create a Stripe Checkout Session via Supabase Edge Function and redirect the user to Stripe.
3. After successful payment, Stripe redirects to a /success page. Handle cancellations with a /canceled page.
4. Create a webhook Edge Function at /functions/v1/stripe-webhook that listens for checkout.session.completed, customer.subscription.updated, and customer.subscription.deleted events.
5. Store subscription status in a Supabase "subscriptions" table linked to the user.
6. Add feature gating: check subscription tier before allowing access to premium features. Show an upgrade prompt for free users.
7. Add a billing management page where users can view their plan, next billing date, and click to open Stripe Customer Portal.

DATABASE:
- "subscriptions" table: id, user_id (FK to auth.users), stripe_customer_id, stripe_subscription_id, plan_tier, status, current_period_end, created_at, updated_at
- RLS: Users can only read their own subscription row.

ENVIRONMENT VARIABLES:
- STRIPE_SECRET_KEY
- STRIPE_WEBHOOK_SECRET
- STRIPE_PRICE_ID_PRO_MONTHLY
- STRIPE_PRICE_ID_PRO_ANNUAL
- STRIPE_PRICE_ID_ENTERPRISE_MONTHLY
- STRIPE_PRICE_ID_ENTERPRISE_ANNUAL

SECURITY:
- Verify Stripe webhook signatures before processing events.
- Never expose secret keys to the client.
- Use RLS so users cannot access other users' subscription data.