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/rankings/page.tsx

@@ -5,6 +5,7 @@
 
 import { fetchRankings, fetchFilters, fetchMetrics } from '@/lib/api';
 import { RankingsView } from '@/components/RankingsView';
+import { absoluteUrl } from '@/lib/site';
 import type { Metadata } from 'next';
 
 interface RankingsPageProps {
@@ -20,6 +21,9 @@ export const metadata: Metadata = {
   title: 'School Rankings',
   description: 'Top-ranked schools by SATs and GCSE performance across England',
   keywords: 'school rankings, top schools, best schools, KS2 rankings, KS4 rankings, school league tables',
+  // Param forms (?metric=&local_authority=&year=&phase=) collapse here for
+  // now. W3 replaces them with real indexable paths.
+  alternates: { canonical: absoluteUrl('/rankings') },
 };
 
 // Dynamic via searchParams; remove force-dynamic so internal data fetches