build(deps): bump fastapi-users from 14.0.1 to 15.0.1 in /backend

Bumps [fastapi-users](https://github.com/fastapi-users/fastapi-users) from 14.0.1 to 15.0.1.
- [Release notes](https://github.com/fastapi-users/fastapi-users/releases)
- [Commits](fastapi-users/fastapi-users@v14.0.1...v15.0.1)

---
updated-dependencies:
- dependency-name: fastapi-users
  dependency-version: 15.0.1
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]