Fix scrub_staging test isolation: use TransactionTestCase

TestCase wraps each test in a savepoint — when scrub_staging deletes
all django_session rows inside that savepoint, the rollback restores
any sessions committed by earlier tests (e.g. force_login in
test_paths_return_200). Those restored rows then leaked into the e2e
live-server tests, causing intermittent Session.MultipleObjectsReturned
errors.

TransactionTestCase flushes the DB before each test instead of using
savepoints, giving scrub_staging a clean slate and removing the leakage.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_scrub_staging.py

@@ -2,12 +2,18 @@ from datetime import timedelta
 
 from django.contrib.sessions.models import Session as DjangoSession
 from django.core.management import call_command
-from django.test import TestCase
+from django.test import TransactionTestCase
 from django.utils.timezone import now
 from django_q.models import Schedule
 
 
-class ScrubStagingTest(TestCase):
+class ScrubStagingTest(TransactionTestCase):
+    # TransactionTestCase flushes the DB before each test instead of wrapping
+    # in a savepoint. Required here because scrub_staging deletes all sessions
+    # — a TestCase savepoint rollback would restore any sessions committed by
+    # earlier tests (e.g. force_login in test_paths_return_200) and leak state
+    # into the e2e live-server tests that follow.
+
     def test_scrub_removes_sessions_and_schedules(self):
         DjangoSession.objects.create(
             session_key="copied-from-prod",
@@ -29,7 +35,6 @@ class ScrubStagingTest(TestCase):
         self.assertEqual(Schedule.objects.count(), 0)
 
     def test_scrub_is_safe_on_empty_database(self):
-        # Should not raise when there is nothing to remove (fresh staging DB).
         call_command("scrub_staging")
 
         self.assertEqual(DjangoSession.objects.count(), 0)