perf(api): batch supplementary queries — one per table, not five per school

get_supplementary_data ran ~5 sequential DB round-trips per URN, so
/api/compare scaled at ~37ms/school (measured on staging: 1 school 155ms,
3 schools 220ms, 6 schools 340ms). get_supplementary_data_batch fetches
each table once with WHERE urn IN (...) and groups in Python, collapsing
5*N round-trips to a constant 5. get_supplementary_data is now a thin
wrapper so the detail endpoint is unchanged; the compare endpoint makes
one batched call. Each table degrades independently on failure.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

backend/app.py

@@ -30,6 +30,7 @@ from .data_loader import (
     load_latest_school_data,
     geocode_single_postcode,
     get_supplementary_data,
+    get_supplementary_data_batch,
     search_schools_typesense,
 )
 from .data_loader import get_data_info as get_db_info
@@ -679,8 +680,10 @@ async def compare_schools(
     db = None
     try:
         db = database.SessionLocal()
+        # One query per table for all schools, not ~5 queries per school.
+        batch = get_supplementary_data_batch(db, urn_list)
         for urn in urn_list:
-            supp = get_supplementary_data(db, urn)
+            supp = batch.get(urn, {})
             supplementary_by_urn[urn] = {
                 key: supp.get(key, default)
                 for key, default in _EMPTY_SUPPLEMENTARY.items()