[add]: refactor/clean up

app/salary_analytics/helpers/data_checks.py

@@ -0,0 +1,12 @@
+from fastapi import HTTPException
+from app.salary_analytics.core.state import state
+
+
+def check_data_loaded():
+    """Raise HTTP 400 if no data is loaded into the pipeline."""
+    if state.pipeline.df is None:
+        raise HTTPException(
+            status_code=400,
+            detail="No data loaded. Please load data first using the /load-data endpoint."
+        )
+    return True