Bump multer from 1.4.5-lts.1 to 2.1.0 in /backend

Bumps [multer](https://github.com/expressjs/multer) from 1.4.5-lts.1 to 2.1.0.
- [Release notes](https://github.com/expressjs/multer/releases)
- [Changelog](https://github.com/expressjs/multer/blob/main/CHANGELOG.md)
- [Commits](expressjs/multer@v1.4.5-lts.1...v2.1.0)

---
updated-dependencies:
- dependency-name: multer
  dependency-version: 2.1.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/package-lock.json

@@ -16,7 +16,7 @@
     "express": "^5.2.0",
     "fs": "^0.0.1-security",
     "glob": "^11.1.0",
-    "multer": "^1.4.5-lts.1",
+    "multer": "^2.1.0",
     "pixelmatch": "^5.3.0",
     "pngjs": "^7.0.0",
     "resize-img": "^2.0.0",