chore(deps): bump pypdf from 4.2.0 to 4.3.1 in /backend

Bumps [pypdf](https://github.com/py-pdf/pypdf) from 4.2.0 to 4.3.1.
- [Release notes](https://github.com/py-pdf/pypdf/releases)
- [Changelog](https://github.com/py-pdf/pypdf/blob/main/CHANGELOG.md)
- [Commits](https://github.com/py-pdf/pypdf/compare/4.2.0...4.3.1)

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

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

backend/requirements.txt

@@ -41,7 +41,7 @@ langchain-chroma==0.1.2
 fake-useragent==1.5.1
 chromadb==0.5.4
 sentence-transformers==3.0.1
-pypdf==4.2.0
+pypdf==4.3.1
 docx2txt==0.8
 python-pptx==0.6.23
 unstructured==0.15.0