build(deps): bump unstructured from 0.15.7 to 0.15.9 in /backend

Bumps [unstructured](https://github.com/Unstructured-IO/unstructured) from 0.15.7 to 0.15.9.
- [Release notes](https://github.com/Unstructured-IO/unstructured/releases)
- [Changelog](https://github.com/Unstructured-IO/unstructured/blob/main/CHANGELOG.md)
- [Commits](https://github.com/Unstructured-IO/unstructured/compare/0.15.7...0.15.9)

---
updated-dependencies:
- dependency-name: unstructured
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

backend/requirements.txt

@@ -44,7 +44,7 @@ sentence-transformers==3.0.1
 pypdf==4.3.1
 docx2txt==0.8
 python-pptx==1.0.0
-unstructured==0.15.7
+unstructured==0.15.9
 nltk==3.9.1
 Markdown==3.7
 pypandoc==1.13