♻️(documents) store document file in object storage in a folder

We will need to store more than a file for a document: multiple languages,
images, etc. For this, the document ID should be a folder and the content
a file in this folder.

src/backend/core/tests/test_models_documents.py

@@ -47,6 +47,12 @@ def test_models_documents_title_max_length():
         factories.DocumentFactory(title="a" * 256)
 
 
+def test_models_documents_file_key():
+    """The file key should be built from the instance uuid."""
+    document = factories.DocumentFactory(id="9531a5f1-42b1-496c-b3f4-1c09ed139b3c")
+    assert document.file_key == "9531a5f1-42b1-496c-b3f4-1c09ed139b3c/file"
+
+
 # get_abilities