refactor(detail): fetch national averages on the server

Both detail views fetched /api/national-averages in a useEffect, so the
England-comparison deltas popped in after hydration and every section that
uses them was pinned to the client. The page now fetches it in parallel with
the school details (backend-cached 1h, degrades to null) and passes it down.

Removes one client round-trip per detail page and unblocks the section
extraction.

Characterization tests pass unmodified; only the render helper changed.

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

nextjs-app/__tests__/support/renderSchoolDetail.tsx

@@ -6,9 +6,8 @@
  * suite may change — the characterization assertions passing unmodified across
  * that rewrite is the proof that behaviour was preserved.
  *
- * National averages are currently fetched client-side via useEffect, so this
- * helper stubs global.fetch. Once they arrive as a server-supplied prop the
- * stub goes away; the tests use findBy* queries so they pass either way.
+ * National averages now arrive as a server-supplied prop rather than a client
+ * fetch, so no fetch stub is needed.
  */
 
 import { render } from '@testing-library/react';
@@ -24,20 +23,16 @@ function withProviders(ui: ReactNode) {
   return <ComparisonProvider>{ui}</ComparisonProvider>;
 }
 
-function stubNationalAveragesFetch() {
-  global.fetch = jest.fn((url: any) =>
-    String(url).includes('national-averages')
-      ? Promise.resolve({ ok: true, json: () => Promise.resolve(nationalAveragesFixture) })
-      : Promise.resolve({ ok: false, json: () => Promise.resolve({}) }),
-  ) as unknown as typeof fetch;
-}
-
 export function renderSchoolDetail(fixture: any) {
-  stubNationalAveragesFetch();
-  return render(withProviders(<SchoolDetailView {...fixture} />));
+  return render(
+    withProviders(<SchoolDetailView {...fixture} nationalAvg={nationalAveragesFixture} />),
+  );
 }
 
 export function renderSecondarySchoolDetail(fixture: any) {
-  stubNationalAveragesFetch();
-  return render(withProviders(<SecondarySchoolDetailView {...fixture} />));
+  return render(
+    withProviders(
+      <SecondarySchoolDetailView {...fixture} nationalAvg={nationalAveragesFixture} />,
+    ),
+  );
 }

nextjs-app/app/school/[slug]/page.tsx

@@ -4,11 +4,12 @@
  * URL format: /school/138267-school-name-here
  */
 
-import { fetchSchoolDetails, fetchSchools } from '@/lib/api';
+import { fetchSchoolDetails, fetchSchools, fetchNationalAverages } from '@/lib/api';
 import { notFound, redirect } from 'next/navigation';
 import { SchoolDetailView } from '@/components/SchoolDetailView';
 import { SecondarySchoolDetailView } from '@/components/SecondarySchoolDetailView';
 import { parseSchoolSlug, schoolUrl } from '@/lib/utils';
+import type { NationalAverages } from '@/lib/types';
 import type { Metadata } from 'next';
 
 /**
@@ -124,10 +125,18 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
     notFound();
   }
 
-  // Fetch school data
+  // Fetch school data. National averages feed the England-comparison deltas
+  // across most sections; fetching them here rather than in a client effect
+  // keeps those sections server-renderable and puts the deltas in the initial
+  // HTML. They are supplementary, so they degrade to null rather than 404ing
+  // the page.
   let data;
+  let nationalAvg: NationalAverages | null = null;
   try {
-    data = await fetchSchoolDetails(urn);
+    [data, nationalAvg] = await Promise.all([
+      fetchSchoolDetails(urn),
+      fetchNationalAverages().catch(() => null),
+    ]);
   } catch (error) {
     console.error(`Failed to fetch school ${urn}:`, error);
     notFound();
@@ -193,6 +202,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
           admissions={admissions ?? null}
           deprivation={deprivation ?? null}
           finance={finance ?? null}
+          nationalAvg={nationalAvg}
         />
       ) : (
         <SchoolDetailView
@@ -205,6 +215,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
           admissionsHistory={admissions_history ?? []}
           deprivation={deprivation ?? null}
           finance={finance ?? null}
+          nationalAvg={nationalAvg}
         />
       )}
     </>

nextjs-app/components/SchoolDetailView.tsx

@@ -70,11 +70,15 @@ interface SchoolDetailViewProps {
   admissionsHistory: SchoolAdmissions[];
   deprivation: SchoolDeprivation | null;
   finance: SchoolFinance | null;
+  /** Fetched on the server so the England-comparison deltas are in the
+   *  initial HTML; null when the endpoint is unavailable. */
+  nationalAvg: NationalAverages | null;
 }
 
 export function SchoolDetailView({
   schoolInfo, yearlyData, absenceData,
   ofsted, census, admissions, admissionsHistory, deprivation, finance,
+  nationalAvg,
 }: SchoolDetailViewProps) {
   const router = useRouter();
   const { addSchool, removeSchool, isSelected } = useComparison();
@@ -169,15 +173,6 @@ export function SchoolDetailView({
   const isSecondary = phase.toLowerCase().includes('secondary') || isAllThrough;
   const isPrimary = !isSecondary;
 
-  // National averages (fetched dynamically so they stay current)
-  const [nationalAvg, setNationalAvg] = useState<NationalAverages | null>(null);
-  useEffect(() => {
-    fetch('/api/national-averages')
-      .then(r => r.ok ? r.json() : null)
-      .then(data => { if (data) setNationalAvg(data); })
-      .catch(() => {});
-  }, []);
-
   const primaryAvg = nationalAvg?.primary ?? {};
   const secondaryAvg = nationalAvg?.secondary ?? {};
 

nextjs-app/components/SecondarySchoolDetailView.tsx

@@ -70,11 +70,15 @@ interface SecondarySchoolDetailViewProps {
   admissions: SchoolAdmissions | null;
   deprivation: SchoolDeprivation | null;
   finance: SchoolFinance | null;
+  /** Fetched on the server so the England-comparison deltas are in the
+   *  initial HTML; null when the endpoint is unavailable. */
+  nationalAvg: NationalAverages | null;
 }
 
 export function SecondarySchoolDetailView({
   schoolInfo, yearlyData,
   ofsted, census, admissions, deprivation, finance, absenceData,
+  nationalAvg,
 }: SecondarySchoolDetailViewProps) {
   const router = useRouter();
   // Hero map — the "View on map" link opens its fullscreen view.
@@ -88,15 +92,6 @@ export function SecondarySchoolDetailView({
 
   const latestResults = yearlyData.length > 0 ? yearlyData[yearlyData.length - 1] : null;
 
-  const [nationalAvg, setNationalAvg] = useState<NationalAverages | null>(null);
-
-  useEffect(() => {
-    fetch('/api/national-averages')
-      .then(r => r.ok ? r.json() : null)
-      .then(data => { if (data) setNationalAvg(data); })
-      .catch(() => {});
-  }, []);
-
   const secondaryAvg = nationalAvg?.secondary ?? {};
 
   // GIAS OfficialSixthForm flag; missing (pipeline not yet re-run) => false.