feat(utils): add buildOfstedListBadge helper and fetchNationalAverages

- Add ofsted_framework field to School type
- Add OfstedListBadge interface and buildOfstedListBadge pure function to utils.ts
- Add fetchNationalAverages API function that calls GET /api/national-averages
- Add test suite for buildOfstedListBadge (all 6 new tests pass)

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

nextjs-app/lib/api.ts

@@ -15,6 +15,7 @@ import type {
   RankingsParams,
   APIError,
   LAaveragesResponse,
+  NationalAverages,
 } from './types';
 
 // ============================================================================
@@ -261,6 +262,26 @@ export async function fetchLAaverages(
   return handleResponse<LAaveragesResponse>(response);
 }
 
+/**
+ * Fetch official DfE KS2 national averages (primary) and computed KS4 secondary averages.
+ * Returns latest year snapshot plus per-year history for chart reference lines.
+ */
+export async function fetchNationalAverages(
+  options: RequestInit = {}
+): Promise<NationalAverages> {
+  const url = `${API_BASE_URL}/national-averages`;
+
+  const response = await fetch(url, {
+    ...options,
+    next: {
+      revalidate: 3600,
+      ...options.next,
+    },
+  });
+
+  return handleResponse<NationalAverages>(response);
+}
+
 /**
  * Fetch database statistics and info
  */