Add Stripe payment integration for AI subscriptions

Implement subscription-based AI access with 250 generations/month at $5/month or $50/year.

Changes:
- Backend: Stripe service, payment routes, webhook handlers, generation tracking
- Frontend: Upgrade page with pricing, payment success/cancel pages, UI prompts
- Database: Add subscription fields to users, payments table, migrations
- Config: Stripe env vars to .env.example, docker-compose.prod.yml, PRODUCTION.md
- Tests: Payment route tests, component tests, subscription hook tests

Users without AI access see upgrade prompts; subscribers see remaining generation count.

docs/PRODUCTION.md

@@ -204,6 +204,45 @@ The frontend is built inside Docker using the `KABOOT_DOMAIN` and `AUTH_DOMAIN`
 
 The `setup-prod.sh` script sets these domain variables automatically.
 
+### Stripe Payments Configuration (Optional)
+
+To enable paid AI access subscriptions, configure Stripe:
+
+```env
+# Stripe API Keys (get from https://dashboard.stripe.com/apikeys)
+STRIPE_SECRET_KEY=sk_live_...        # Use sk_test_... for testing
+STRIPE_WEBHOOK_SECRET=whsec_...      # From webhook endpoint configuration
+STRIPE_PRICE_ID_MONTHLY=price_...    # Monthly subscription price ID
+STRIPE_PRICE_ID_YEARLY=price_...     # Yearly subscription price ID
+```
+
+#### Stripe Dashboard Setup
+
+1. **Create a Product** in [Stripe Dashboard](https://dashboard.stripe.com/products):
+   - Name: "Kaboot AI Pro"
+   - Description: "250 AI quiz generations per month"
+
+2. **Add Pricing**:
+   - Monthly: $5.00/month (recurring)
+   - Yearly: $50.00/year (recurring)
+   - Copy the Price IDs (start with `price_`)
+
+3. **Configure Webhook**:
+   - Go to [Developers > Webhooks](https://dashboard.stripe.com/webhooks)
+   - Add endpoint: `https://your-domain.com/api/payments/webhook`
+   - Select events:
+     - `checkout.session.completed`
+     - `customer.subscription.updated`
+     - `customer.subscription.deleted`
+     - `invoice.paid`
+     - `invoice.payment_failed`
+   - Copy the Signing Secret (starts with `whsec_`)
+
+4. **Test with Stripe CLI** (optional, for local development):
+   ```bash
+   stripe listen --forward-to localhost:3001/api/payments/webhook
+   ```
+
 ## Docker Compose Files
 
 The project includes pre-configured compose files: