fix: use manage.py instead of python -m django in test workflow

python -m django doesn't set DJANGO_SETTINGS_MODULE, causing
ImproperlyConfigured error. manage.py sets it correctly.

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

Author: vpetersson

.github/workflows/test-runner.yml

@@ -55,9 +55,9 @@ jobs:
           docker compose exec anthias-test \
             bash ./bin/prepare_test_environment.sh
           docker compose exec anthias-test \
-            python -m django migrate --run-syncdb
+            ./manage.py migrate --run-syncdb
           docker compose exec anthias-test \
-            python -m django collectstatic --noinput --clear
+            ./manage.py collectstatic --noinput --clear
 
       - name: Run Python integration tests
         run: |