Update pydantic requirement from >=2.9.2 to >=2.13.3 in /backend

Updates the requirements on [pydantic](https://github.com/pydantic/pydantic) to permit the latest version.
- [Release notes](https://github.com/pydantic/pydantic/releases)
- [Changelog](https://github.com/pydantic/pydantic/blob/main/HISTORY.md)
- [Commits](pydantic/pydantic@v2.9.2...v2.13.3)

---
updated-dependencies:
- dependency-name: pydantic
  dependency-version: 2.13.3
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -96,7 +96,7 @@ dependencies = [
     "PyJWT>=2.8.0",
     "cryptography>=43.0.0",
     "openai>=1.52.0",
-    "pydantic>=2.9.2",
+    "pydantic>=2.13.3",
     "gunicorn>=23.0.0",
     "tinycss2>=1.4.0",
     "pypdf>=5.1.0",