fix: import datetime in email_service.py to fix NameError, add Stripe emails unit tests

services/email_service.py

@@ -8,6 +8,7 @@ Configuration is resolved from settings JSON file, then env vars.
 import os
 import logging
 import smtplib
+from datetime import datetime
 from email.mime.text import MIMEText
 from email.mime.multipart import MIMEMultipart
 from typing import Optional