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

nextjs-app/__tests__/app/metadata.test.ts

@@ -0,0 +1,23 @@
+import { metadata as homeMetadata } from '@/app/page';
+import { metadata as rankingsMetadata } from '@/app/rankings/page';
+import { metadata as admissionsMetadata } from '@/app/admissions/page';
+
+describe('canonical URLs', () => {
+  it('the homepage canonicalises to the bare root', () => {
+    // page.tsx reads eleven search params. Without this, every filter
+    // combination is a crawlable near-duplicate of the one page we want to
+    // rank for "compare schools".
+    expect(homeMetadata.alternates?.canonical)
+      .toBe('https://www.schoolcompare.co.uk/');
+  });
+
+  it('rankings canonicalises to the bare path', () => {
+    expect(rankingsMetadata.alternates?.canonical)
+      .toBe('https://www.schoolcompare.co.uk/rankings');
+  });
+
+  it('admissions canonicalises to the bare path', () => {
+    expect(admissionsMetadata.alternates?.canonical)
+      .toBe('https://www.schoolcompare.co.uk/admissions');
+  });
+});