🔊(backend) remove email addresses from invitation failure logs

Email addresses are PII and should not appear in technical or
error logs.

Sanitize logging to avoid exposing sensitive user data when
invitation sending fails.

src/backend/core/services/invitation.py

@@ -58,5 +58,5 @@ class InvitationService:
             try:
                 email.send()
             except smtplib.SMTPException as e:
-                logger.error("invitation to %s was not sent: %s", emails, e)
+                logger.error("invitations were not sent: %s", e)
                 raise InvitationError("Could not send invitation") from e