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

backend/app.py

@@ -48,7 +48,9 @@ PHASE_GROUPS: dict[str, set[str]] = {
     "all-through": {"all-through"},
 }
 
-BASE_URL = "https://schoolcompare.co.uk"
+# Must match SITE_URL in nextjs-app/lib/site.ts. The apex 301s to www, and a
+# sitemap <loc> that redirects wastes a crawl on every URL it lists.
+BASE_URL = "https://www.schoolcompare.co.uk"
 MAX_SLUG_LENGTH = 60
 
 # In-memory sitemap cache