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/tests/test_sixth_form_flag.py

@@ -95,3 +95,84 @@ def test_detail_payload_includes_flag(client):
     resp = client.get("/api/schools/100002")
     assert resp.status_code == 200, resp.text
     assert resp.json()["school_info"]["has_sixth_form"] is True
+
+
+def test_detail_payload_serializes_numpy_bool(monkeypatch):
+    """Once the pipeline has run, has_sixth_form is a real bool dtype column
+    (dbt not_null test guarantees no NULLs), so row access yields
+    numpy.bool_ rather than a Python bool. convert_to_native must handle it —
+    otherwise FastAPI's jsonable_encoder raises ValueError and the detail
+    endpoint 500s (C2)."""
+    from backend import app as app_module
+
+    df = _schools_df()
+    # Drop the row with a None flag — this fixture models the post-pipeline
+    # state where the column is a genuine, fully-populated bool dtype.
+    df = df[df["has_sixth_form"].notna()].reset_index(drop=True)
+    df["has_sixth_form"] = df["has_sixth_form"].astype(bool)
+    assert df["has_sixth_form"].dtype == bool
+
+    monkeypatch.setattr(app_module, "load_school_data", lambda: df)
+    monkeypatch.setattr(app_module, "get_supplementary_data", lambda db, urn: {})
+    client = TestClient(app_module.app, raise_server_exceptions=False)
+
+    resp = client.get("/api/schools/100002")
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["school_info"]["has_sixth_form"] is True
+
+
+class _FakeProgrammingError(Exception):
+    """Stand-in for sqlalchemy.exc.ProgrammingError wrapping psycopg2's
+    UndefinedColumn, without needing a real DB connection to construct one."""
+
+
+def test_load_school_data_survives_missing_has_sixth_form_column(monkeypatch):
+    """Real prod state until the nightly pipeline first rebuilds the mart:
+    marts.dim_school lacks has_sixth_form entirely. The first query raises
+    UndefinedColumn; load_school_data_as_dataframe must retry without the
+    column (synthesizing it as None) rather than swallow the error and
+    return (and then have load_school_data cache) an empty DataFrame (C1)."""
+    import sqlalchemy.exc
+    from backend import data_loader
+
+    data_loader._df_cache = None
+    data_loader._df_latest_cache = None
+
+    good_df = pd.DataFrame(
+        [
+            {
+                "urn": 1,
+                "school_name": "Fallback School",
+                "school_type": "Academy",
+                "has_sixth_form": None,
+            }
+        ]
+    )
+    calls = []
+
+    def fake_read_sql(query, con):
+        calls.append(query)
+        if len(calls) == 1:
+            raise sqlalchemy.exc.ProgrammingError(
+                "SELECT ...",
+                None,
+                Exception(
+                    "(psycopg2.errors.UndefinedColumn) column s.has_sixth_form "
+                    "does not exist"
+                ),
+            )
+        return good_df.copy()
+
+    monkeypatch.setattr(data_loader.pd, "read_sql", fake_read_sql)
+
+    try:
+        df = data_loader.load_school_data_as_dataframe()
+    finally:
+        data_loader._df_cache = None
+        data_loader._df_latest_cache = None
+
+    assert len(calls) == 2, "must retry with the no-sixth-form query variant"
+    assert calls[1] is data_loader._MAIN_QUERY_NO_SIXTH_FORM
+    assert not df.empty
+    assert "has_sixth_form" in df.columns
+    assert df["has_sixth_form"].iloc[0] is None