feat(blog): add the blog index, post pages, RSS and content sitemap

The rendering split is dictated by CI building with no database.

/blog, /blog/rss.xml and /content-sitemap.xml have no dynamic params, so
Next prerenders them at build time and the build fails on a missing
Payload secret — caught here, not on staging. They are force-dynamic
instead: one indexed query against Postgres on the same Docker network,
and a newly published post appears immediately rather than waiting on a
revalidation. /blog/[slug] keeps ISR, because with no
generateStaticParams there is nothing to prerender; it is generated on
first request and cached, which is exactly what the collection's
afterChange hook exists to invalidate.

RichText takes `converters`, not `blocks`, in Payload 3.88, and the
default converters must be spread or every paragraph and heading loses
its renderer and the body comes out empty.

BlogPosting references the Person and Organization by @id rather than
repeating them, so every post and the About page resolve to one author
entity instead of declaring several people with the same name.

/sitemap.xml is proxied from FastAPI, which knows nothing about Payload,
so the Next-owned URLs get their own sitemap and robots.txt lists both.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017YmbBhr8s7GusjDE12hrZM

nextjs-app/app/(frontend)/content-sitemap.xml/route.ts

@@ -0,0 +1,52 @@
+/*
+ * A second sitemap for the URLs Next owns.
+ *
+ * /sitemap.xml is proxied from FastAPI (app/(frontend)/sitemap.xml), which
+ * knows nothing about Payload — the backend and frontend ship as separate
+ * images. Rather than teach it, the Next-owned URLs get their own sitemap and
+ * robots.txt lists both.
+ */
+import { getCachedPayload } from '@/lib/payload';
+import { absoluteUrl } from '@/lib/site';
+
+/*
+ * Dynamic, not ISR.
+ *
+ * This route has no dynamic params, so Next prerenders it at build time — and
+ * CI builds the image with no database reachable, which fails the build. It is
+ * a single indexed query against Postgres on the same Docker network, so
+ * rendering per request is cheap, and it means a newly published post appears
+ * here immediately rather than waiting on a revalidation.
+ */
+export const dynamic = 'force-dynamic';
+
+export async function GET() {
+  const payload = await getCachedPayload();
+  const { docs } = await payload.find({
+    collection: 'posts',
+    where: { _status: { equals: 'published' } },
+    sort: '-publishedAt',
+    limit: 500,
+    depth: 0,
+  });
+
+  const urls: Array<{ loc: string; lastmod: string | null }> = [
+    { loc: absoluteUrl('/about'), lastmod: null },
+    { loc: absoluteUrl('/blog'), lastmod: null },
+    ...docs.map((post) => ({
+      loc: absoluteUrl(`/blog/${post.slug}`),
+      lastmod: new Date(String(post.updatedAt ?? post.publishedAt)).toISOString(),
+    })),
+  ];
+
+  const xml = `<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+${urls.map(({ loc, lastmod }) =>
+  `  <url><loc>${loc}</loc>${lastmod ? `<lastmod>${lastmod}</lastmod>` : ''}</url>`,
+).join('\n')}
+</urlset>`;
+
+  return new Response(xml, {
+    headers: { 'Content-Type': 'application/xml; charset=utf-8' },
+  });
+}