fix(phase-04): use MagicMock(spec=MinIOBackend) in test_daily_export_download

Plain MagicMock() failed the isinstance(backend, MinIOBackend) guard in
download_daily_export(), returning 404. spec=MinIOBackend sets __class__
so isinstance passes and the mock path executes correctly.

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

backend/tests/test_audit.py

@@ -313,6 +313,7 @@ async def test_daily_exports_list(async_client, admin_user):
 async def test_daily_export_download(async_client, admin_user):
     """GET /api/admin/audit-log/daily-exports/{date} returns CSV bytes with Content-Disposition (D-16)"""
     from unittest.mock import MagicMock, patch
+    from storage.minio_backend import MinIOBackend
 
     fake_csv = b"id,event_type,user_id\n1,document.uploaded,abc\n"
 
@@ -324,7 +325,7 @@ async def test_daily_export_download(async_client, admin_user):
     mock_client = MagicMock()
     mock_client.get_object.return_value = mock_response
 
-    mock_backend = MagicMock()
+    mock_backend = MagicMock(spec=MinIOBackend)
     mock_backend._client = mock_client
 
     with patch("api.audit.get_storage_backend", return_value=mock_backend):