build(deps): bump pypandoc from 1.13 to 1.15 in /backend

Bumps [pypandoc](https://github.com/JessicaTegner/pypandoc) from 1.13 to 1.15.
- [Release notes](https://github.com/JessicaTegner/pypandoc/releases)
- [Changelog](https://github.com/JessicaTegner/pypandoc/blob/master/release.md)
- [Commits](https://github.com/JessicaTegner/pypandoc/compare/v1.13...v1.15)

---
updated-dependencies:
- dependency-name: pypandoc
  dependency-version: '1.15'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -67,7 +67,7 @@ python-pptx==1.0.0
 unstructured==0.16.17
 nltk==3.9.1
 Markdown==3.7
-pypandoc==1.13
+pypandoc==1.15
 pandas==2.2.3
 openpyxl==3.1.5
 pyxlsb==1.0.10