style: fix Ruff issues (F401 unused import, E741 ambiguous var)

nl2sql/pipeline.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 import traceback
-from dataclasses import dataclass, asdict
+from dataclasses import dataclass
 from typing import Dict, Any, Optional, List
 
 from nl2sql.types import StageResult

ui/benchmark_app.py

@@ -15,7 +15,7 @@ if not result_files:
     st.stop()
 
 file = st.selectbox("Select benchmark file", result_files)
-rows = [json.loads(l) for l in open(file)]
+rows = [json.loads(line) for line in open(file)]
 df = pd.DataFrame(rows)
 
 # 2. Summary metrics