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-lock.json

@@ -27,7 +27,7 @@
         "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",
@@ -5317,9 +5317,9 @@
       }
     },
     "node_modules/zod": {
-      "version": "3.25.76",
-      "resolved": "https://registry.npmjs.org/zod/-/zod-3.25.76.tgz",
-      "integrity": "sha512-gzUt/qt81nXsFGKIFcC3YnfEAx5NkunCfnDlvuBSSFS02bcXu4Lmea0AFIUwbLWxWPx3d9p8S5QoaujKcNQxcQ==",
+      "version": "4.4.3",
+      "resolved": "https://registry.npmjs.org/zod/-/zod-4.4.3.tgz",
+      "integrity": "sha512-ytENFjIJFl2UwYglde2jchW2Hwm4GJFLDiSXWdTrJQBIN9Fcyp7n4DhxJEiWNAJMV1/BqWfW/kkg71UDcHJyTQ==",
       "license": "MIT",
       "funding": {
         "url": "https://github.com/sponsors/colinhacks"