fix(seo): declare a canonical on every route

The homepage read eleven search params and declared no canonical, so every
filter combination was a crawlable near-duplicate of the page we most want to
rank. Rankings and admissions declared none either.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

e2e/tests/journeys.spec.ts

@@ -1600,3 +1600,45 @@ test('the sitemap submits no Welsh or overseas school', async ({ page }) => {
   expect(xml).not.toContain('/school/401559');
   expect(xml).not.toContain('/school/402426');
 });
+
+/*
+ * Canonical URLs (spec 2026-08-20, W1).
+ *
+ * Every indexable route declares exactly one canonical, on the www host, with
+ * no query string. The homepage's eleven search params filter a result set
+ * rather than making a new document, so they all collapse onto "/".
+ */
+const CANONICAL_ROUTES: Array<[string, string]> = [
+  ['/', 'https://www.schoolcompare.co.uk/'],
+  ['/rankings', 'https://www.schoolcompare.co.uk/rankings'],
+  ['/admissions', 'https://www.schoolcompare.co.uk/admissions'],
+];
+
+for (const [path, expected] of CANONICAL_ROUTES) {
+  test(`${path} declares exactly one canonical, on the www host`, async ({ page }) => {
+    await page.goto(path);
+    const hrefs = await page.locator('link[rel="canonical"]').evaluateAll(
+      (els) => els.map((e) => e.getAttribute('href')));
+    expect(hrefs, `${path} should declare one canonical`).toHaveLength(1);
+    expect(hrefs[0]).toBe(expected);
+  });
+}
+
+test('a filtered homepage still canonicalises to the bare root', async ({ page }) => {
+  await page.goto('/?search=primary&phase=primary&sort=name&page=2');
+  const href = await page.locator('link[rel="canonical"]').first()
+    .getAttribute('href');
+  expect(href).toBe('https://www.schoolcompare.co.uk/');
+});
+
+test('a school page canonicalises to its own slug on the www host', async ({ page }) => {
+  const res = await page.request.get('/api/schools?search=primary&per_page=1');
+  expect(res.ok()).toBeTruthy();
+  const [first] = (await res.json()).schools ?? [];
+  expect(first, 'no school available').toBeTruthy();
+
+  await page.goto(`/school/${first.urn}-x`);
+  const href = await page.locator('link[rel="canonical"]').first()
+    .getAttribute('href');
+  expect(href).toMatch(/^https:\/\/www\.schoolcompare\.co\.uk\/school\/\d+-/);
+});