♻️(backend) return the full document in the duplicate response

The duplicate endpoint used to respond with only `{"id": ...}`. It now
returns the complete duplicated document representation, consistent
with the other document detail endpoints, so the frontend doesn't have
to make a follow-up request to get the new document's data.

This required setting `is_favorite` explicitly on the duplicated
document before serializing it: it is normally set by the
`annotate_is_favorite` queryset method, which the newly created
document never goes through. Being a read-only serializer field, it
was silently dropped from the response instead of raising an error. A
document can't be a favorite right after being created, so it is set
to `False` directly.

src/backend/core/api/viewsets.py

@@ -1349,10 +1349,16 @@ class DocumentViewSet(
             document=duplicated_document,
         )
 
-        return drf_response.Response(
-            {"id": str(duplicated_document.id)}, status=status.HTTP_201_CREATED
+        # Set the `is_favorite` attribute to False for the duplicated document, as it
+        # cannot be a favorite immediately after creation.
+        duplicated_document.is_favorite = False
+
+        serializer = serializers.DocumentSerializer(
+            duplicated_document, context=self.get_serializer_context()
         )
 
+        return drf_response.Response(serializer.data, status=status.HTTP_201_CREATED)
+
     def _duplicate_document(
         self,
         document_to_duplicate,