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.

server/package.json

@@ -21,6 +21,7 @@
     "jwks-rsa": "^3.1.0",
     "multer": "^2.0.2",
     "officeparser": "^6.0.4",
+    "stripe": "^20.2.0",
     "uuid": "^11.0.5"
   },
   "devDependencies": {