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/robots.ts

@@ -17,6 +17,9 @@ export default function robots(): MetadataRoute.Robots {
         disallow: ['/api/', '/_next/', '/admin/', '/cms-api/'],
       },
     ],
-    sitemap: absoluteUrl('/sitemap.xml'),
+    // Two sitemaps: /sitemap.xml is proxied from FastAPI and carries the
+    // school corpus; /content-sitemap.xml is Next-owned and carries /about
+    // and the blog. The backend knows nothing about Payload.
+    sitemap: [absoluteUrl('/sitemap.xml'), absoluteUrl('/content-sitemap.xml')],
   };
 }