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/data_loader.py

@@ -3,11 +3,14 @@ Data loading module — reads from marts.* tables built by dbt.
 Provides efficient queries with caching.
 """
 
+import logging
+
 import pandas as pd
 import numpy as np
 from typing import Optional, Dict, Tuple, List
 import requests
 from sqlalchemy import text
+import sqlalchemy.exc
 from sqlalchemy.orm import Session
 
 from .config import settings
@@ -215,11 +218,36 @@ _MAIN_QUERY = text("""
     ORDER BY s.school_name, p.year
 """)
 
+# Fallback used when marts.dim_school predates the has_sixth_form column
+# (i.e. the nightly dbt pipeline hasn't rebuilt the mart yet on this DB).
+# Keeps the column present as NULL so downstream code — including the
+# app.py fallback branch — behaves as designed instead of KeyError-ing.
+_MAIN_QUERY_NO_SIXTH_FORM = text(
+    str(_MAIN_QUERY).replace("s.has_sixth_form,", "NULL AS has_sixth_form,")
+)
+assert "NULL AS has_sixth_form" in str(_MAIN_QUERY_NO_SIXTH_FORM), (
+    "expected replacement of 's.has_sixth_form,' to have taken effect"
+)
+
 
 def load_school_data_as_dataframe() -> pd.DataFrame:
     """Load all school + KS2 data as a pandas DataFrame."""
     try:
         df = pd.read_sql(_MAIN_QUERY, engine)
+    except sqlalchemy.exc.ProgrammingError as exc:
+        if "has_sixth_form" not in str(exc):
+            print(f"Warning: Could not load school data from marts: {exc}")
+            return pd.DataFrame()
+        logging.getLogger(__name__).warning(
+            "marts.dim_school is missing has_sixth_form (pipeline hasn't "
+            "rebuilt the mart yet on this DB) — retrying without it: %s",
+            exc,
+        )
+        try:
+            df = pd.read_sql(_MAIN_QUERY_NO_SIXTH_FORM, engine)
+        except Exception as exc2:
+            print(f"Warning: Could not load school data from marts: {exc2}")
+            return pd.DataFrame()
     except Exception as exc:
         print(f"Warning: Could not load school data from marts: {exc}")
         return pd.DataFrame()