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

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](pydantic/pydantic@v2.12.5...v2.13.4)

---
updated-dependencies:
- dependency-name: pydantic
  dependency-version: 2.13.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -2,7 +2,7 @@ fastapi==0.135.1
 uvicorn[standard]==0.41.0
 sqlalchemy==2.0.47
 alembic==1.18.4
-pydantic==2.12.5
+pydantic==2.13.4
 pydantic-settings==2.13.1
 psycopg2-binary==2.9.11
 asyncpg==0.31.0