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/app/page.tsx

@@ -3,6 +3,7 @@
  * Main landing page with school search and browsing
  */
 
+import { absoluteUrl } from '@/lib/site';
 import type { Metadata } from 'next';
 import { fetchSchools, fetchFilters, fetchDataInfo } from '@/lib/api';
 import { formatAcademicYear } from '@/lib/utils';
@@ -35,6 +36,10 @@ interface HomePageProps {
 export const metadata: Metadata = {
   title: { absolute: 'schoolcompare | Compare every school in England' },
   description: 'Search and compare school performance across England',
+  // This page reads eleven search params. They filter a result set; they do
+  // not make a new document. Collapsing every combination onto "/" stops the
+  // homepage competing with itself for its own head terms.
+  alternates: { canonical: absoluteUrl('/') },
 };
 
 // The page reads searchParams, which makes rendering dynamic by default.