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/models.py

@@ -251,7 +251,10 @@ class CensusBenchmark(Base):
 
 
 class Ks4NationalAverage(Base):
-    """Computed national KS4 averages (from our dataset) — one row per year."""
+    """Official DfE KS4 national headline averages — one row per academic year.
+
+    gcse_grade_91_pct has no official national series and is always NULL.
+    """
     __tablename__ = "fact_ks4_national_averages"
     __table_args__ = MARTS