🔒️(backend) fix email disclosure in room invitation endpoint

Prevent invited participants from seeing each other's email
addresses when sending room invitations.

Ensure invitations are sent with proper isolation to avoid
mass PII disclosure.

This mitigates risks of email harvesting, spam, and phishing
through the platform.

src/backend/core/services/invitation.py

@@ -4,7 +4,7 @@ import smtplib
 from logging import getLogger
 
 from django.conf import settings
-from django.core.mail import send_mail
+from django.core.mail import EmailMultiAlternatives
 from django.template.loader import render_to_string
 from django.utils.translation import get_language, override
 from django.utils.translation import gettext_lazy as _
@@ -45,15 +45,18 @@ class InvitationService:
                 )
             )  # Force translation
 
+            email = EmailMultiAlternatives(
+                subject=subject,
+                body=msg_plain,
+                from_email=settings.EMAIL_FROM,
+                to=[],
+                bcc=emails,
+            )
+
+            email.attach_alternative(msg_html, "text/html")
+
             try:
-                send_mail(
-                    subject,
-                    msg_plain,
-                    settings.EMAIL_FROM,
-                    emails,
-                    html_message=msg_html,
-                    fail_silently=False,
-                )
+                email.send()
             except smtplib.SMTPException as e:
                 logger.error("invitation to %s was not sent: %s", emails, e)
                 raise InvitationError("Could not send invitation") from e