spec: update api consistency

Align optimistic locking conflict payload, update websocket docs, and adjust tests.

backend/tests/test_concurrency_reliability.py

@@ -88,7 +88,11 @@ class TestOptimisticLocking:
         )
 
         assert response.status_code == 409
-        assert "conflict" in response.json().get("detail", "").lower() or "version" in response.json().get("detail", "").lower()
+        detail = response.json().get("detail")
+        assert isinstance(detail, dict)
+        assert detail.get("error") == "conflict"
+        assert detail.get("current_version") == 5
+        assert detail.get("provided_version") == 1
 
     def test_update_without_version_succeeds(self, client, admin_token, db):
         """Test that update without version (for backward compatibility) still works."""