♻️(backend) force saving invitation email in lowercase

We want to enforce that invitation email are saved in lower case.

src/backend/core/api/serializers.py

@@ -740,6 +740,8 @@ class InvitationSerializer(serializers.ModelSerializer):
         if self.instance is None:
             attrs["issuer"] = user
 
+        attrs["email"] = attrs["email"].lower()
+
         return attrs
 
     def validate_role(self, role):