fix(07): add status field to _doc_to_dict + regression test

DocumentCard classification_failed badge requires doc.status from the
API list endpoint. _doc_to_dict was omitting the field; this fix adds
it and adds a regression test that would have caught the omission.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/storage.py

@@ -61,6 +61,7 @@ def _doc_to_dict(doc: Document, topic_names: list) -> dict:
         "topics": topic_names,
         "created_at": doc.created_at.isoformat() if doc.created_at else None,
         "classified_at": doc.updated_at.isoformat() if doc.status == "classified" else None,
+        "status": doc.status,
     }