head --> root to avoid confusion, prevent root deletion

[ci skip]

src/documents/tests/test_document_model.py

@@ -78,8 +78,8 @@ class TestDocument(TestCase):
             empty_trash([document.pk])
             self.assertEqual(mock_unlink.call_count, 2)
 
-    def test_delete_head_deletes_versions(self) -> None:
-        head = Document.objects.create(
+    def test_delete_root_deletes_versions(self) -> None:
+        root = Document.objects.create(
             correspondent=Correspondent.objects.create(name="Test0"),
             title="Head",
             content="content",
@@ -87,15 +87,15 @@ class TestDocument(TestCase):
             mime_type="application/pdf",
         )
         Document.objects.create(
-            head_version=head,
-            correspondent=head.correspondent,
+            root_document=root,
+            correspondent=root.correspondent,
             title="Version",
             content="content",
             checksum="checksum2",
             mime_type="application/pdf",
         )
 
-        head.delete()
+        root.delete()
 
         self.assertEqual(Document.objects.count(), 0)
         self.assertEqual(Document.deleted_objects.count(), 2)