✨(backend) add duplicate action to the document API endpoint

We took this opportunity to refactor the way access is controlled on
media attachments. We now add the media key to a list on the document
instance each time a media is uploaded to a document. This list is
passed along when a document is duplicated, allowing us to grant
access to readers on the new document, even if they don't have or
lost access to the original document.

We also propose an option to reproduce the same access rights on the
duplicate document as what was in place on the original document.
This can be requested by passing the "with_accesses=true" option in
the query string.

The tricky point is that we need to extract attachment keys from the
existing documents and set them on the new "attachments" field that is
now used to track access rights on media files.

src/backend/core/tests/migrations/test_migrations_0020_remove_is_public_add_field_attachments_and_duplicated_from.py

@@ -0,0 +1,55 @@
+import base64
+import uuid
+
+from django.core.files.base import ContentFile
+from django.core.files.storage import default_storage
+
+import pytest
+import y_py
+
+from core import models
+
+
+@pytest.mark.django_db
+def test_populate_attachments_on_all_documents(migrator):
+    """Test that the migration populates attachments on existing documents."""
+    old_state = migrator.apply_initial_migration(
+        ("core", "0019_alter_user_language_default_to_null")
+    )
+    OldDocument = old_state.apps.get_model("core", "Document")
+
+    old_doc_without_attachments = OldDocument.objects.create(
+        title="Doc without attachments", depth=1, path="0000002"
+    )
+    old_doc_with_attachments = OldDocument.objects.create(
+        title="Doc with attachments", depth=1, path="0000001"
+    )
+
+    # Create document content with an image
+    file_key = f"{old_doc_with_attachments.id!s}/file"
+    ydoc = y_py.YDoc()  # pylint: disable=no-member
+    image_key = f"{old_doc_with_attachments.id!s}/attachments/{uuid.uuid4()!s}.png"
+    with ydoc.begin_transaction() as txn:
+        xml_fragment = ydoc.get_xml_element("document-store")
+        xml_fragment.push_xml_element(txn, "image").set_attribute(
+            txn, "src", f"http://localhost/media/{image_key:s}"
+        )
+    update = y_py.encode_state_as_update(ydoc)  # pylint: disable=no-member
+    base64_content = base64.b64encode(update).decode("utf-8")
+    bytes_content = base64_content.encode("utf-8")
+    content_file = ContentFile(bytes_content)
+    default_storage.save(file_key, content_file)
+
+    # Apply the migration
+    new_state = migrator.apply_tested_migration(
+        ("core", "0020_remove_is_public_add_field_attachments_and_duplicated_from")
+    )
+    NewDocument = new_state.apps.get_model("core", "Document")
+
+    new_doc_with_attachments = NewDocument.objects.get(pk=old_doc_with_attachments.pk)
+    new_doc_without_attachments = NewDocument.objects.get(
+        pk=old_doc_without_attachments.pk
+    )
+
+    assert new_doc_without_attachments.attachments == []
+    assert new_doc_with_attachments.attachments == [image_key]