feat(seo): split the sitemap into a per-family index

Search Console reports coverage per submitted sitemap, so one file per page
family is what will make W2's location pages measurable when they land. The
index's lastmod is generation time, which is the correct semantic there —
unlike on a <url>, where it would be a claim we cannot support.

Children sit under /sitemaps/ because Next only treats a whole bracketed path
segment as dynamic; a route folder named sitemap-[...parts] would be read as a
literal static segment and never match. Confirmed by the build output, which
lists /sitemaps/[...parts] as a dynamic route.

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

nextjs-app/app/sitemap.xml/route.ts

@@ -1,32 +1,8 @@
-/**
- * Runtime proxy for /sitemap.xml → the FastAPI backend's generated sitemap.
- *
- * Like the /api/* proxy, this reads FASTAPI_URL at request time rather than
- * baking the backend host into the build, so one image works in every
- * environment. robots.ts points crawlers here.
- */
-
-import { NextResponse } from 'next/server';
+import { proxySitemap } from '@/lib/sitemapProxy';
 
 export const dynamic = 'force-dynamic';
 export const runtime = 'nodejs';
 
-function backendOrigin(): string {
-  const base = process.env.FASTAPI_URL || process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000/api';
-  return base.replace(/\/api$/, '');
-}
-
 export async function GET() {
-  let upstream: Response;
-  try {
-    upstream = await fetch(`${backendOrigin()}/sitemap.xml`, { cache: 'no-store' });
-  } catch {
-    return new NextResponse('Sitemap temporarily unavailable', { status: 502 });
-  }
-
-  const body = await upstream.text();
-  return new NextResponse(body, {
-    status: upstream.status,
-    headers: { 'content-type': upstream.headers.get('content-type') || 'application/xml' },
-  });
+  return proxySitemap('/sitemap.xml');
 }

nextjs-app/app/sitemaps/[...parts]/route.ts

@@ -0,0 +1,24 @@
+import { NextResponse } from 'next/server';
+import { proxySitemap } from '@/lib/sitemapProxy';
+
+export const dynamic = 'force-dynamic';
+export const runtime = 'nodejs';
+
+/**
+ * Children are /sitemaps/static.xml and /sitemaps/schools-{n}.xml. The name is
+ * validated here rather than passed through, so this route cannot be used to
+ * reach arbitrary backend paths.
+ */
+const CHILD = /^(static|schools-\d+)\.xml$/;
+
+export async function GET(
+  _request: Request,
+  { params }: { params: Promise<{ parts: string[] }> },
+) {
+  const { parts } = await params;
+  const name = parts.join('/');
+  if (!CHILD.test(name)) {
+    return new NextResponse('Not found', { status: 404 });
+  }
+  return proxySitemap(`/sitemaps/${name}`);
+}

nextjs-app/lib/sitemapProxy.ts

@@ -0,0 +1,29 @@
+/**
+ * Runtime proxy for the sitemap family → the FastAPI backend.
+ *
+ * Like the /api/* proxy, this reads FASTAPI_URL at request time rather than
+ * baking the backend host into the build, so one image works in every
+ * environment. robots.ts points crawlers at /sitemap.xml, which is the index;
+ * the index names children under /sitemaps/, which land on the same proxy.
+ */
+import { NextResponse } from 'next/server';
+
+function backendOrigin(): string {
+  const base = process.env.FASTAPI_URL || process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000/api';
+  return base.replace(/\/api$/, '');
+}
+
+export async function proxySitemap(path: string): Promise<NextResponse> {
+  let upstream: Response;
+  try {
+    upstream = await fetch(`${backendOrigin()}${path}`, { cache: 'no-store' });
+  } catch {
+    return new NextResponse('Sitemap temporarily unavailable', { status: 502 });
+  }
+
+  const body = await upstream.text();
+  return new NextResponse(body, {
+    status: upstream.status,
+    headers: { 'content-type': upstream.headers.get('content-type') || 'application/xml' },
+  });
+}