build(deps): bump zod from 3.25.76 to 4.4.3 in /backend

* build(deps): bump zod from 3.25.76 to 4.4.3 in /backend

Bumps [zod](https://github.com/colinhacks/zod) from 3.25.76 to 4.4.3.
- [Release notes](https://github.com/colinhacks/zod/releases)
- [Commits](https://github.com/colinhacks/zod/compare/v3.25.76...v4.4.3)

---
updated-dependencies:
- dependency-name: zod
  dependency-version: 4.4.3
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

* fix: adapt backend validation for zod v4

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Daniel Volz <mail@danielvolz.org>

backend/package.json

@@ -36,7 +36,7 @@
     "nodemailer": "^8.0.7",
     "openid-client": "^6.8.4",
     "sharp": "^0.34.5",
-    "zod": "^3.23.8"
+    "zod": "^4.4.3"
   },
   "devDependencies": {
     "@biomejs/biome": "^2.4.14",