Add schema-driven db:migrate for SQLite and Postgres

- Parse schema files to detect expected columns per table
- Add missing columns automatically when schema is updated
- Add payment_request to sponsors schema
- Add db:migrate script to root package.json

Made-with: Cursor

package.json

@@ -7,6 +7,7 @@
     "build:frontend": "cd frontend && npm run build",
     "start": "cd backend && npm start",
     "dev:backend": "cd backend && npm run dev",
-    "dev:frontend": "cd frontend && npm run dev"
+    "dev:frontend": "cd frontend && npm run dev",
+    "db:migrate": "cd backend && npm run migrate"
   }
 }