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-lock.json

@@ -18,6 +18,7 @@
         "jwks-rsa": "^3.1.0",
         "multer": "^2.0.2",
         "officeparser": "^6.0.4",
+        "stripe": "^20.2.0",
         "uuid": "^11.0.5"
       },
       "devDependencies": {
@@ -1454,7 +1455,6 @@
       "resolved": "https://registry.npmjs.org/express/-/express-4.22.1.tgz",
       "integrity": "sha512-F2X8g9P1X7uCPZMA3MVf9wcTqlyNp7IhH5qPCI0izhaOIYXaW9L535tGA3qmjRzpH+bZczqq7hVKxTR4NWnu+g==",
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "accepts": "~1.3.8",
         "array-flatten": "1.1.1",
@@ -2896,6 +2896,26 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/stripe": {
+      "version": "20.2.0",
+      "resolved": "https://registry.npmjs.org/stripe/-/stripe-20.2.0.tgz",
+      "integrity": "sha512-m8niTfdm3nPP/yQswRWMwQxqEUcTtB3RTJQ9oo6NINDzgi7aPOadsH/fPXIIfL1Sc5+lqQFKSk7WiO6CXmvaeA==",
+      "license": "MIT",
+      "dependencies": {
+        "qs": "^6.14.1"
+      },
+      "engines": {
+        "node": ">=16"
+      },
+      "peerDependencies": {
+        "@types/node": ">=16"
+      },
+      "peerDependenciesMeta": {
+        "@types/node": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/strtok3": {
       "version": "6.3.0",
       "resolved": "https://registry.npmjs.org/strtok3/-/strtok3-6.3.0.tgz",
@@ -3205,7 +3225,6 @@
       "resolved": "https://registry.npmjs.org/zod/-/zod-3.25.76.tgz",
       "integrity": "sha512-gzUt/qt81nXsFGKIFcC3YnfEAx5NkunCfnDlvuBSSFS02bcXu4Lmea0AFIUwbLWxWPx3d9p8S5QoaujKcNQxcQ==",
       "license": "MIT",
-      "peer": true,
       "funding": {
         "url": "https://github.com/sponsors/colinhacks"
       }