🔒️(backend) remove personal data from email failure logs

Fix code that accidentally exposed personal email addresses in logs during
email sending failures. Modify logging to remove identifying information
to protect user privacy while still providing useful debugging context.

Original code was inspired by Docs.

src/backend/core/recording/event/notification.py

@@ -83,7 +83,7 @@ class NotificationService:
                     fail_silently=False,
                 )
             except smtplib.SMTPException as exception:
-                logger.error("notification to %s was not sent: %s", emails, exception)
+                logger.error("notification could not be sent: %s", exception)
                 return False
 
         return True