fix: backend/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-ZIPP-7430899

Author: snyk-bot

backend/requirements.txt

@@ -16,3 +16,4 @@ pandas>=2.0.0
 APScheduler>=3.10.4
 litellm>=1.50.0
 langchain-litellm>=0.3.0
+zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability