chore(deps): bump pydantic from 2.12.5 to 2.13.4 in /backend (#244)

Bumps [pydantic](https://github.com/pydantic/pydantic) from 2.12.5 to 2.13.4.
- [Release notes](https://github.com/pydantic/pydantic/releases)
- [Changelog](https://github.com/pydantic/pydantic/blob/v2.13.4/HISTORY.md)
- [Commits](https://github.com/pydantic/pydantic/compare/v2.12.5...v2.13.4)

---
updated-dependencies:
- dependency-name: pydantic
  dependency-version: 2.13.4
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Christian Krakau-Louis <christianlouis@gmail.com>

backend/requirements.txt

@@ -1,7 +1,7 @@
 # Core Framework
 fastapi==0.135.3  # Updated: Fixed starlette DoS vulnerabilities (was 0.109.1); pulls in starlette>=1.0.0
 uvicorn[standard]==0.43.0
-pydantic==2.12.5
+pydantic==2.13.4
 pydantic-settings==2.13.1
 
 # Database