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/utils.ts

@@ -570,3 +570,49 @@ export function buildSchoolSummary(
 
   return parts.join(', ') + '.';
 }
+
+// ─── List-level Ofsted badge ──────────────────────────────────────────────────
+
+export interface OfstedListBadge {
+  /** Display text for the badge (e.g. "Outstanding · 2023", "Report Card · 2025") */
+  label: string;
+  /** CSS module class key — one of: ofsted1 | ofsted2 | ofsted3 | ofsted4 | ofstedRc | ofstedPending */
+  cssClass: string;
+}
+
+/**
+ * Build the Ofsted badge for a school card in the list/map view.
+ * Three states:
+ *   - OEIF school (ofsted_grade set): grade word + year, colour-keyed
+ *   - ReportCard school (ofsted_framework === 'ReportCard'): "Report Card · YYYY" in purple
+ *   - No inspection: "Not yet inspected" in grey
+ */
+export function buildOfstedListBadge(school: {
+  ofsted_grade?: number | null;
+  ofsted_date?: string | null;
+  ofsted_framework?: string | null;
+}): OfstedListBadge {
+  const year = school.ofsted_date
+    ? new Date(school.ofsted_date).getFullYear()
+    : null;
+  const yearStr = year ? ` · ${year}` : '';
+
+  if (school.ofsted_grade) {
+    const labels: Record<number, string> = {
+      1: 'Outstanding',
+      2: 'Good',
+      3: 'Req. Improvement',
+      4: 'Inadequate',
+    };
+    return {
+      label: `${labels[school.ofsted_grade]}${yearStr}`,
+      cssClass: `ofsted${school.ofsted_grade}`,
+    };
+  }
+
+  if (school.ofsted_framework === 'ReportCard') {
+    return { label: `Report Card${yearStr}`, cssClass: 'ofstedRc' };
+  }
+
+  return { label: 'Not yet inspected', cssClass: 'ofstedPending' };
+}