fix(data): official DfE KS4 national headline averages; drop mislabelled computed means

New ees_ks4_national stream ingests the EES 'National characteristics
summary data' series (England, state-funded, all pupils). The old mart's
unweighted school means were 7-15 points off every headline measure and
produced an impossible national Progress 8 (-0.27). The API's computed
fallback is gone too: the footnote calls these figures official, so an
unbuilt mart now yields an empty series, never a stand-in.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

backend/app.py

@@ -823,11 +823,11 @@ def _national_averages_payload(df: pd.DataFrame) -> dict:
     /api/compare.
 
     Both series are persisted marts computed at import time: official DfE
-    KS2 figures (fact_ks2_national_averages) and dataset-computed KS4
-    averages (fact_ks4_national_averages) — the API never aggregates the
-    performance dataframe per request. If the KS4 mart hasn't been built
-    yet (deploy lands before the next DAG run), fall back to computing the
-    latest year only — a single-year scan, never the historical loop.
+    KS2 figures (fact_ks2_national_averages) and official DfE KS4 figures
+    (fact_ks4_national_averages) — the API never aggregates the performance
+    dataframe per request. If the KS4 mart hasn't been built yet, the
+    secondary series is empty — never a computed stand-in, because the UI
+    labels these figures as official DfE data.
     """
     if df.empty:
         return {"primary": {}, "secondary": {}}
@@ -867,23 +867,6 @@ def _national_averages_payload(df: pd.DataFrame) -> dict:
     primary_by_year = {r.year: _row_metrics(r, _KS2_NATIONAL_METRICS) for r in ks2_rows}
     secondary_by_year = {r.year: _row_metrics(r, _KS4_NATIONAL_METRICS) for r in ks4_rows}
 
-    if not any(secondary_by_year.values()):
-        # KS4 mart missing/empty: compute the latest year only.
-        df_latest = df[df["year"] == latest_year]
-        sec = (
-            df_latest[df_latest["attainment_8_score"].notna()]
-            if "attainment_8_score" in df_latest.columns
-            else df_latest.iloc[0:0]
-        )
-        vals = {}
-        for col in _KS4_NATIONAL_METRICS:
-            if col in sec.columns:
-                v = sec[col].dropna()
-                if len(v) > 0:
-                    vals[col] = round(float(v.mean()), 2)
-        if vals:
-            secondary_by_year[latest_year] = vals
-
     all_years = sorted(set(primary_by_year) | set(secondary_by_year))
     by_year = [
         {