fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-REQUESTS-6928867
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-7267250
- https://snyk.io/vuln/SNYK-PYTHON-WERKZEUG-6035177
- https://snyk.io/vuln/SNYK-PYTHON-WERKZEUG-6808933
- https://snyk.io/vuln/SNYK-PYTHON-ZIPP-7430899

requirements.txt

@@ -3,4 +3,8 @@ Flask~=3.0.0
 Flask-Login~=0.6.0
 argon2-cffi~=23.1.0
 stripe~=10.10.0
-zappa~=0.59.0
+zappa~=0.59.0
+requests>=2.32.2 # not directly required, pinned by Snyk to avoid a vulnerability
+urllib3>=2.2.2 # not directly required, pinned by Snyk to avoid a vulnerability
+werkzeug>=3.0.3 # not directly required, pinned by Snyk to avoid a vulnerability
+zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability