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/lib/jsonld.ts

@@ -32,3 +32,47 @@ export function organizationJsonLd() {
     logo: absoluteUrl('/icon-512.png'),
   } as const;
 }
+
+interface PostSummary {
+  title: string;
+  slug: string;
+  excerpt: string;
+  publishedAt: string;
+}
+
+/**
+ * References the Person and Organization by @id rather than repeating them, so
+ * search engines resolve every post and the About page to the one author
+ * entity. Repeating the shape would declare several people with one name.
+ */
+export function blogPostingJsonLd(post: PostSummary) {
+  return {
+    '@type': 'BlogPosting',
+    headline: post.title,
+    description: post.excerpt,
+    url: absoluteUrl(`/blog/${post.slug}`),
+    datePublished: post.publishedAt,
+    author: { '@id': `${SITE_URL}/about#tudor` },
+    publisher: { '@id': `${SITE_URL}#organization` },
+  } as const;
+}
+
+export function breadcrumbJsonLd(post: PostSummary) {
+  return {
+    '@type': 'BreadcrumbList',
+    itemListElement: [
+      {
+        '@type': 'ListItem',
+        position: 1,
+        name: 'Blog',
+        item: absoluteUrl('/blog'),
+      },
+      {
+        '@type': 'ListItem',
+        position: 2,
+        name: post.title,
+        item: absoluteUrl(`/blog/${post.slug}`),
+      },
+    ],
+  } as const;
+}