build(deps): bump pillow from 11.1.0 to 11.2.1 in /backend

Bumps [pillow](https://github.com/python-pillow/Pillow) from 11.1.0 to 11.2.1.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/main/CHANGES.rst)
- [Commits](https://github.com/python-pillow/Pillow/compare/11.1.0...11.2.1)

---
updated-dependencies:
- dependency-name: pillow
  dependency-version: 11.2.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -82,7 +82,7 @@ sentencepiece
 soundfile==0.13.1
 azure-ai-documentintelligence==1.0.0
 
-pillow==11.1.0
+pillow==11.2.1
 opencv-python-headless==4.11.0.86
 rapidocr-onnxruntime==1.4.4
 rank-bm25==0.2.2