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

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

backend/app.py

@@ -416,10 +416,14 @@ async def get_schools(
         df_latest = df_latest[df_latest["gender"].str.lower() == gender.lower()]
     if admissions_policy:
         df_latest = df_latest[df_latest["admissions_policy"].str.lower() == admissions_policy.lower()]
-    if has_sixth_form == "yes":
-        df_latest = df_latest[df_latest["age_range"].str.contains("18", na=False)]
-    elif has_sixth_form == "no":
-        df_latest = df_latest[~df_latest["age_range"].str.contains("18", na=False)]
+    # GIAS OfficialSixthForm flag (dim_school.has_sixth_form). NULL (flag not
+    # yet populated by the pipeline) is treated as "no sixth form".
+    if has_sixth_form in ("yes", "no"):
+        if "has_sixth_form" in df_latest.columns:
+            flag = df_latest["has_sixth_form"].eq(True)
+        else:  # DB predates the pipeline re-run — fall back to age range
+            flag = df_latest["age_range"].str.contains("18", na=False)
+        df_latest = df_latest[flag if has_sixth_form == "yes" else ~flag]
 
     # Include key result metrics for display on cards
     location_cols = ["latitude", "longitude"]
@@ -596,6 +600,7 @@ async def get_school_details(request: Request, urn: int):
             "address": latest.get("address", ""),
             "religious_denomination": latest.get("religious_denomination", ""),
             "age_range": latest.get("age_range", ""),
+            "has_sixth_form": latest.get("has_sixth_form"),
             "latitude": latest.get("latitude"),
             "longitude": latest.get("longitude"),
             "phase": latest.get("phase"),