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.

docker-compose.prod.yml

@@ -101,6 +101,10 @@ services:
       CORS_ORIGIN: ${CORS_ORIGIN}
       LOG_REQUESTS: ${LOG_REQUESTS:-true}
       GEMINI_API_KEY: ${GEMINI_API_KEY:-}
+      STRIPE_SECRET_KEY: ${STRIPE_SECRET_KEY:-}
+      STRIPE_WEBHOOK_SECRET: ${STRIPE_WEBHOOK_SECRET:-}
+      STRIPE_PRICE_ID_MONTHLY: ${STRIPE_PRICE_ID_MONTHLY:-}
+      STRIPE_PRICE_ID_YEARLY: ${STRIPE_PRICE_ID_YEARLY:-}
     volumes:
       - kaboot-data:/data
     networks: