Bump openai from 4.12.1 to 5.16.0 in /backend

Bumps [openai](https://github.com/openai/openai-node) from 4.12.1 to 5.16.0.
- [Release notes](https://github.com/openai/openai-node/releases)
- [Changelog](https://github.com/openai/openai-node/blob/master/CHANGELOG.md)
- [Commits](openai/openai-node@v4.12.1...v5.16.0)

---
updated-dependencies:
- dependency-name: openai
  dependency-version: 5.16.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]