feat(pipeline): derive dim_school.has_sixth_form from GIAS flag

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/models.py

@@ -24,6 +24,7 @@ class DimSchool(Base):
     religious_character = Column(String(100))
     gender = Column(String(20))
     age_range = Column(String(20))
+    has_sixth_form = Column(Boolean)
     capacity = Column(Integer)
     total_pupils = Column(Integer)
     headteacher_name = Column(String(200))