🚨(backend) follow Ruff 2024.2 style introduced in v0.3.0

We recently updated Ruff from 0.2.2 to v0.3, which introduced
Ruff 2024.2 style. This new style updated Ruff formatter's behavior,
making our make lint command fails.

Ruff 2024.2 style add a blank line after the module docstring.
Please take a look at Ruff ChangeLog to get more info.

src/backend/core/api/viewsets.py

@@ -1,4 +1,5 @@
 """API endpoints"""
+
 from django.contrib.postgres.search import TrigramSimilarity
 from django.db.models import Func, Max, OuterRef, Prefetch, Q, Subquery, Value