chore(deps): bump langfuse from 3.6.1 to 3.6.2 in /backend

Bumps [langfuse](https://github.com/langfuse/langfuse) from 3.6.1 to 3.6.2.
- [Release notes](https://github.com/langfuse/langfuse/releases)
- [Commits](langfuse/langfuse@v3.6.1...v3.6.2)

---
updated-dependencies:
- dependency-name: langfuse
  dependency-version: 3.6.2
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -55,7 +55,7 @@ openai==2.1.0
 purgatory==3.0.1  # Circuit breaker for resilient API calls
 
 # Observability
-langfuse==3.6.1
+langfuse==3.6.2
 sentry-sdk[fastapi]==2.39.0
 
 # Authentication