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/components/blog/CalloutBlock.module.css

@@ -0,0 +1,21 @@
+.caveat,
+.note {
+  border-left: 3px solid var(--brand);
+  background: var(--brand-bg);
+  padding: 1rem 1.15rem;
+  margin: 1.75rem 0;
+  border-radius: 0 8px 8px 0;
+}
+
+.note {
+  border-left-color: var(--border-strong);
+  background: var(--bg-secondary);
+}
+
+.body {
+  font-family: var(--font-ui);
+  font-size: 0.95rem;
+  line-height: 1.65;
+  color: var(--text-primary);
+  margin: 0;
+}

nextjs-app/components/blog/CalloutBlock.tsx

@@ -0,0 +1,14 @@
+import styles from './CalloutBlock.module.css';
+
+/**
+ * Renders the Callout block from blocks/Callout.ts. The "caveat" tone is the
+ * one that matters: it is how a post says what a number does not show, in
+ * context, rather than burying it in a closing paragraph.
+ */
+export function CalloutBlock({ tone, body }: { tone: string; body: string }) {
+  return (
+    <aside className={tone === 'caveat' ? styles.caveat : styles.note}>
+      <p className={styles.body}>{body}</p>
+    </aside>
+  );
+}