fix(api): serve DfE's LA averages for "vs LA avg" (H2)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/models.py

@@ -344,6 +344,18 @@ class Ks4NationalAverage(Base):
     gcse_grade_91_pct = Column(Float)
 
 
+class Ks4LaAverage(Base):
+    """Official DfE KS4 local-authority averages (all state-funded schools) —
+    one row per academic year and LA. la_code is the GIAS LA code."""
+    __tablename__ = "fact_ks4_la_averages"
+    __table_args__ = MARTS
+
+    year = Column(Integer, primary_key=True)
+    la_code = Column(Integer, primary_key=True)
+    la_name = Column(String)
+    attainment_8_score = Column(Float)
+
+
 class Ks2NationalAverage(Base):
     """Official DfE KS2 national headline averages — one row per academic year."""
     __tablename__ = "fact_ks2_national_averages"