🐛(backend) fix flaky test by clarifying user ordering

On the user search API by similarity, we had a flaky test because
2 users had the same similarity score. Adding a secondary ordering
field makes ordering deterministic between users who share the same
similarity score.

src/backend/core/api/viewsets.py

@@ -175,7 +175,7 @@ class UserViewSet(
                 threshold = 0.6 if "@" in query else 0.1
 
                 queryset = queryset.filter(similarity__gt=threshold).order_by(
-                    "-similarity"
+                    "-similarity", "email"
                 )
 
         return queryset