fix(checkpointer): drop duplicate CubepiHitlAnswer import in postgres test

Commit 37979d4 re-added CubepiHitlAnswer to the import block while it
was already present, tripping ruff F811 (redefinition) and breaking CI.

Author: claude

tests/checkpointer/test_postgres.py

@@ -10,7 +10,6 @@
 def test_models_import() -> None:
     from cubepi.checkpointer.postgres.models import (
         EXPECTED_SCHEMA_VERSION,
-        CubepiHitlAnswer,
         PARTITION_COUNT,
         CubepiHitlAnswer,
         CubepiMessage,