chore(deps): bump langfuse from 2.38.0 to 2.39.2 in /backend

Bumps [langfuse](https://github.com/langfuse/langfuse) from 2.38.0 to 2.39.2.
- [Release notes](https://github.com/langfuse/langfuse/releases)
- [Commits](https://github.com/langfuse/langfuse/commits

)

---
updated-dependencies:
- dependency-name: langfuse
  dependency-type: direct:production
  update-type: version-update:semver-minor
...
Signed-off-by: dependabot[bot] <support@github.com>

backend/requirements.txt

@@ -65,7 +65,7 @@ PyJWT[crypto]==2.8.0
 authlib==1.3.1
 
 black==24.4.2
-langfuse==2.38.0
+langfuse==2.39.2
 youtube-transcript-api==0.6.2
 pytube==15.0.0