fix(seo): canonicalise on the www host, which is the one that serves 200

The apex 301s to www at Cloudflare, but metadataBase, the school-page
canonical, robots.txt's Sitemap: line and the sitemap's own <loc> entries all
named the apex. Every one of those pointed Google at a redirect.

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

nextjs-app/app/layout.tsx

@@ -5,6 +5,7 @@ import { Navigation } from '@/components/Navigation';
 import { Footer } from '@/components/Footer';
 import { ComparisonToast } from '@/components/ComparisonToast';
 import { ComparisonProvider } from '@/context/ComparisonProvider';
+import { SITE_URL } from '@/lib/site';
 import './globals.css';
 
 // Manrope carries headings and key messaging — the guideline's "friendly,
@@ -57,12 +58,12 @@ export const metadata: Metadata = {
   // No `icons` key on purpose: setting it here would override the file
   // conventions. app/icon.svg and app/apple-icon.tsx are the source, and
   // app/opengraph-image.tsx supplies og:image and twitter:image.
-  metadataBase: new URL('https://schoolcompare.co.uk'),
+  metadataBase: new URL(SITE_URL),
   openGraph: {
     type: 'website',
     title: 'schoolcompare | Compare School Performance',
     description: 'Compare primary and secondary school SATs and GCSE performance across England',
-    url: 'https://schoolcompare.co.uk',
+    url: SITE_URL,
     siteName: 'schoolcompare',
   },
   twitter: {

nextjs-app/app/robots.ts

@@ -4,6 +4,7 @@
  */
 
 import { MetadataRoute } from 'next';
+import { absoluteUrl } from '@/lib/site';
 
 export default function robots(): MetadataRoute.Robots {
   return {
@@ -14,6 +15,6 @@ export default function robots(): MetadataRoute.Robots {
         disallow: ['/api/', '/_next/'],
       },
     ],
-    sitemap: 'https://schoolcompare.co.uk/sitemap.xml',
+    sitemap: absoluteUrl('/sitemap.xml'),
   };
 }

nextjs-app/app/school/[slug]/page.tsx

@@ -15,6 +15,7 @@ import {
 } from '@/lib/schoolSections';
 import { parseSchoolSlug, schoolUrl } from '@/lib/utils';
 import type { NationalAverages } from '@/lib/types';
+import { absoluteUrl } from '@/lib/site';
 import type { Metadata } from 'next';
 
 /**
@@ -97,7 +98,7 @@ export async function generateMetadata({ params }: SchoolPageProps): Promise<Met
         title,
         description,
         type: 'website',
-        url: `https://schoolcompare.co.uk${canonicalPath}`,
+        url: absoluteUrl(canonicalPath),
         siteName: 'schoolcompare',
       },
       twitter: {
@@ -106,7 +107,7 @@ export async function generateMetadata({ params }: SchoolPageProps): Promise<Met
         description,
       },
       alternates: {
-        canonical: `https://schoolcompare.co.uk${canonicalPath}`,
+        canonical: absoluteUrl(canonicalPath),
       },
     };
   } catch {