chore(deps): bump unstructured from 0.14.10 to 0.15.0 in /backend

Bumps [unstructured](https://github.com/Unstructured-IO/unstructured) from 0.14.10 to 0.15.0.
- [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/0.14.10...0.15.0)

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

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

backend/requirements.txt

@@ -43,7 +43,7 @@ sentence-transformers==3.0.1
 pypdf==4.2.0
 docx2txt==0.8
 python-pptx==0.6.23
-unstructured==0.14.10
+unstructured==0.15.0
 Markdown==3.6
 pypandoc==1.13
 pandas==2.2.2