implement gunicorn for prod WSGI server

app/requirements.txt

@@ -1,6 +1,9 @@
 # This file is used by pip to install required python packages
 # Usage: pip install -r requirements.txt
 
+# Gunicorn WSGI Server
+gunicorn==20.1.0
+
 # Flask Framework
 click==8.1.3
 Flask==2.2.2