fix(05-09): set storage_backend='minio' in test_celery_task_uses_user_provider

Cloud-aware routing added in 05-09 checks doc.storage_backend; MagicMock
attribute is truthy and != 'minio', so the test was entering the cloud branch
without any mock for get_storage_backend_for_document. Regression: test passed
before 05-09 when _run() had no cloud routing.

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

backend/tests/test_classifier.py

@@ -184,6 +184,7 @@ async def test_celery_task_uses_user_provider(db_session):
     mock_doc.content_type = "text/plain"
     mock_doc.extracted_text = ""
     mock_doc.status = "uploaded"
+    mock_doc.storage_backend = "minio"
 
     mock_user = MagicMock()
     mock_user.ai_provider = "anthropic"