🚨(backend) fix linter warnings

Recent updates of dev/ruff and dev/pylint dependencies led
to new linting warnings.

Pylint 3.2.0 introduced a new check `possibly-used-before-assignment`,
which ensures variables are defined regardless of conditional statements.

Some if/else branches were missing defaults. These have been fixed.

src/backend/core/api/viewsets.py

@@ -1,4 +1,5 @@
 """API endpoints"""
+
 import uuid
 
 from django.conf import settings