fix(api): serialize schools that have no performance rows

Schools without KS2/KS4 results (special post-16 institutions, sixth-form
centres, PRUs, new schools) come back from the marts LEFT JOIN with NaN in
every numeric column. school_info passed those raw pandas values straight
into JSONResponse, which renders with allow_nan=False, so the detail
endpoint 500d and the frontend turned that into a 404 on every such SEO
landing page.

Run school_info values through convert_to_native (the same treatment
yearly_data already gets), add backend unit tests plus a pytest step in PR
checks, and an e2e journey that finds a results-less school via the search
API and asserts its page renders.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

e2e/tests/journeys.spec.ts

@@ -60,6 +60,34 @@ test('school detail page renders name and performance data', async ({ page }) =>
   await expect(page.locator('canvas:visible').first()).toBeVisible({ timeout: 15_000 });
 });
 
+test('school with no performance data still gets a working detail page', async ({ page }) => {
+  // Schools without KS2/KS4 results (special post-16 institutions, sixth-form
+  // centres, PRUs) used to 500 in the API — NaN GIAS fields broke JSON
+  // serialization — which the frontend rendered as a 404 on every such SEO
+  // landing page. Find one via the search API (year === null marks "no
+  // performance rows") and assert its page renders.
+  const candidates: number[] = [];
+  for (const q of ['post 16', 'specialist college', 'sixth form']) {
+    const resp = await page.request.get(
+      `/api/schools?search=${encodeURIComponent(q)}&per_page=20`
+    );
+    if (!resp.ok()) continue;
+    const body = await resp.json();
+    for (const s of body.schools ?? []) {
+      if (s.year === null && s.urn) candidates.push(s.urn);
+    }
+    if (candidates.length) break;
+  }
+  test.skip(candidates.length === 0, 'no results-less school in this dataset');
+
+  const detail = await page.request.get(`/api/schools/${candidates[0]}`);
+  expect(detail.status(), 'detail API must not 500 for a results-less school').toBe(200);
+
+  await page.goto(`/school/${candidates[0]}`);
+  await page.waitForURL(/\/school\/\d+-/); // redirected to canonical slug
+  await expect(page.locator('h1').first()).toBeVisible();
+});
+
 test('school hero map opens fullscreen on mobile without the Fullscreen API', async ({ page }) => {
   // iOS Safari has no Element.requestFullscreen; the map must fall back to a
   // CSS overlay. Simulate that by removing the API before any page script runs.