chore(backend): bump zod-openapi in /services/backend

Bumps [zod-openapi](https://github.com/samchungy/zod-openapi) from 5.4.6 to 6.0.0.
- [Release notes](https://github.com/samchungy/zod-openapi/releases)
- [Changelog](https://github.com/samchungy/zod-openapi/blob/master/CHANGELOG.md)
- [Commits](https://github.com/samchungy/zod-openapi/compare/zod-openapi@5.4.6...zod-openapi@6.0.0)

---
updated-dependencies:
- dependency-name: zod-openapi
  dependency-version: 6.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

services/backend/package.json

@@ -49,7 +49,7 @@
     "pug": "^3.0.2",
     "sanitize-html": "^2.17.0",
     "zod": "^4.3.6",
-    "zod-openapi": "^5.4.6"
+    "zod-openapi": "^6.0.0"
   },
   "devDependencies": {
     "@commitlint/cli": "^20.4.3",