feat(api): drive has_sixth_form filter and payloads from GIAS flag

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

backend/data_loader.py

@@ -125,6 +125,7 @@ _MAIN_QUERY = text("""
         s.religious_character  AS religious_denomination,
         s.gender,
         s.age_range,
+        s.has_sixth_form,
         s.admissions_policy,
         s.capacity,
         s.total_pupils         AS gias_total_pupils,