fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-5918878

requirements.txt

@@ -8,4 +8,5 @@ python-dotenv
 sentence-transformers
 unstructured[pdf]
 pypdf
-nltk>=3.8.2 # not directly required, pinned by Snyk to avoid a vulnerability
+nltk>=3.8.2 # not directly required, pinned by Snyk to avoid a vulnerability
+pillow>=10.0.1 # not directly required, pinned by Snyk to avoid a vulnerability