fix(api): survive missing has_sixth_form column and numpy bool serialization

- data_loader.load_school_data_as_dataframe now catches a ProgrammingError
  whose message mentions has_sixth_form (psycopg2 UndefinedColumn) and
  retries with a NULL-AS-has_sixth_form query variant, so the API keeps
  serving data (and the app.py column-fallback branch stays reachable)
  even before the nightly pipeline has rebuilt marts.dim_school.
- utils.convert_to_native now handles numpy.bool_ so GET /api/schools/{urn}
  doesn't 500 once has_sixth_form is a populated bool-dtype column.
- Update the now-stale comment on the app.py age-range fallback branch.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/utils.py

@@ -11,6 +11,8 @@ def convert_to_native(value: Any) -> Any:
     """Convert numpy types to native Python types for JSON serialization."""
     if pd.isna(value):
         return None
+    if isinstance(value, np.bool_):
+        return bool(value)
     if isinstance(value, (np.integer,)):
         return int(value)
     if isinstance(value, (np.floating,)):