perf(api): persist KS4 national averages as a mart; stop per-request aggregation

fact_ks4_national_averages is computed once at dbt build time (covered by
the EES DAG's stg_ees_ks4+ selector). _national_averages_payload now reads
both national-averages marts instead of scanning the performance dataframe
per year on every /api/compare request (~250ms saved per call). Fallback
for the deploy-before-DAG window computes the latest year only.

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

backend/models.py

@@ -231,6 +231,23 @@ class FactFinance(Base):
     premises_cost_pct = Column(Float)
 
 
+class Ks4NationalAverage(Base):
+    """Computed national KS4 averages (from our dataset) — one row per year."""
+    __tablename__ = "fact_ks4_national_averages"
+    __table_args__ = MARTS
+
+    year = Column(Integer, primary_key=True)
+    attainment_8_score = Column(Float)
+    progress_8_score = Column(Float)
+    english_maths_standard_pass_pct = Column(Float)
+    english_maths_strong_pass_pct = Column(Float)
+    ebacc_entry_pct = Column(Float)
+    ebacc_standard_pass_pct = Column(Float)
+    ebacc_strong_pass_pct = Column(Float)
+    ebacc_avg_score = Column(Float)
+    gcse_grade_91_pct = Column(Float)
+
+
 class Ks2NationalAverage(Base):
     """Official DfE KS2 national headline averages — one row per academic year."""
     __tablename__ = "fact_ks2_national_averages"