Initialize the converter backend later to allow proper registration of app

apps/converter/api.py

@@ -12,7 +12,7 @@ from mimetype.api import get_mimetype
 
 from .literals import (DEFAULT_PAGE_NUMBER,
     DEFAULT_ZOOM_LEVEL, DEFAULT_ROTATION, DEFAULT_FILE_FORMAT)
-from . import backend
+from .runtime import backend
 from .literals import (TRANSFORMATION_CHOICES, TRANSFORMATION_RESIZE,
     TRANSFORMATION_ROTATE, TRANSFORMATION_ZOOM, DIMENSION_SEPARATOR,
     FILE_FORMATS)