fix(secondary): expose GIAS total_pupils in school_info API response

The school_info object was missing total_pupils entirely, so the frontend
always fell back to the KS4 exam cohort from yearly_data. Now selects
s.total_pupils (GIAS NumberOfPupils — full school roll) as gias_total_pupils
in the main query and exposes it as total_pupils on school_info.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app.py

@@ -553,6 +553,7 @@ async def get_school_details(request: Request, urn: int):
             "website": latest.get("website"),
             "headteacher_name": latest.get("headteacher_name"),
             "capacity": latest.get("capacity"),
+            "total_pupils": latest.get("gias_total_pupils"),
             "trust_name": latest.get("trust_name"),
             "gender": latest.get("gender"),
         },

backend/data_loader.py

@@ -127,6 +127,7 @@ _MAIN_QUERY = text("""
         s.age_range,
         s.admissions_policy,
         s.capacity,
+        s.total_pupils         AS gias_total_pupils,
         s.headteacher_name,
         s.website,
         s.ofsted_grade,