chore(deps): bump express from 5.1.0 to 5.2.1 in /backend

Bumps [express](https://github.com/expressjs/express) from 5.1.0 to 5.2.1.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](expressjs/express@v5.1.0...v5.2.1)

---
updated-dependencies:
- dependency-name: express
  dependency-version: 5.2.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/package-lock.json

@@ -23,7 +23,7 @@
     "cors": "^2.8.5",
     "csurf": "^1.11.0",
     "dotenv": "^16.5.0",
-    "express": "^5.1.0",
+    "express": "^5.2.1",
     "express-rate-limit": "^8.2.1",
     "express-validator": "^7.3.1",
     "helmet": "^8.1.0",