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)/blog/Blog.module.css

@@ -0,0 +1,76 @@
+.page {
+  max-width: 42rem;
+  margin: 0 auto;
+  padding: 2.5rem 1.25rem 4rem;
+}
+
+.header { margin-bottom: 2.5rem; }
+
+.kicker {
+  font-family: var(--font-ui);
+  font-size: 0.75rem;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  color: var(--brand);
+  margin: 0 0 0.35rem;
+}
+
+.heading {
+  font-family: var(--font-display);
+  font-size: clamp(1.5rem, 4vw, 2rem);
+  font-weight: 700;
+  line-height: 1.2;
+  color: var(--text-primary);
+  margin: 0 0 0.75rem;
+}
+
+.standfirst {
+  font-family: var(--font-ui);
+  font-size: 1.05rem;
+  line-height: 1.65;
+  color: var(--text-secondary);
+  margin: 0;
+}
+
+.list { list-style: none; padding: 0; margin: 0; }
+
+.item {
+  padding: 1.5rem 0;
+  border-top: 1px solid var(--border);
+}
+
+.date {
+  font-family: var(--font-ui);
+  font-size: 0.8rem;
+  color: var(--text-muted);
+  /* Inter's tabular numerals keep a column of dates aligned. */
+  font-variant-numeric: tabular-nums;
+}
+
+.itemTitle {
+  font-family: var(--font-display);
+  font-size: 1.25rem;
+  font-weight: 600;
+  line-height: 1.3;
+  margin: 0.35rem 0 0.5rem;
+}
+
+.itemLink { color: var(--text-primary); text-decoration: none; }
+.itemLink:hover { color: var(--brand); }
+
+.excerpt {
+  font-family: var(--font-ui);
+  font-size: 0.95rem;
+  line-height: 1.65;
+  color: var(--text-secondary);
+  margin: 0;
+}
+
+.empty {
+  font-family: var(--font-ui);
+  color: var(--text-muted);
+}
+
+.link { color: var(--brand); font-weight: 600; }
+.link:hover { color: var(--brand-strong); }