build(deps): bump litellm from 1.61.1 to 1.61.15 in /backend

Bumps [litellm](https://github.com/BerriAI/litellm) from 1.61.1 to 1.61.15.
- [Release notes](https://github.com/BerriAI/litellm/releases)
- [Commits](https://github.com/BerriAI/litellm/commits)

---
updated-dependencies:
- dependency-name: litellm
dependency-type: direct:production
...

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

backend/pyproject.toml

@@ -69,7 +69,7 @@ dependencies = [
     "langchain-core==0.3.34",
     "langchain-text-splitters==0.3.6",
     "langsmith==0.3.6",
-    "litellm==1.61.1",
+    "litellm==1.61.15",
     "lxml==5.3.0",
     "lxml-html-clean==0.4.1",
     "markdown-it-py==3.0.0",