Bump express from 5.1.0 to 5.2.0 in /backend

Bumps [express](https://github.com/expressjs/express) from 5.1.0 to 5.2.0.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](expressjs/express@v5.1.0...v5.2.0)

---
updated-dependencies:
- dependency-name: express
  dependency-version: 5.2.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/package-lock.json

@@ -13,7 +13,7 @@
     "@types/pngjs": "^6.0.4",
     "body-parser": "^2.2.1",
     "cors": "^2.8.5",
-    "express": "^5.1.0",
+    "express": "^5.2.0",
     "fs": "^0.0.1-security",
     "glob": "^11.1.0",
     "multer": "^1.4.5-lts.1",