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/__tests__/payload/collections.test.ts

@@ -33,13 +33,13 @@ describe('posts collection', () => {
     expect(POSTS).toMatch(/access:\s*\{\s*read:\s*\(\)\s*=>\s*true/);
   });
 
-  it('revalidates the blog paths when a post changes or is deleted', () => {
-    // Blog pages are ISR because CI builds with no database. Without these
-    // hooks a published post would not appear until the revalidate window
-    // expired — up to an hour of a writer thinking publishing is broken.
+  it('revalidates the post page when a post changes or is deleted', () => {
+    // /blog/[slug] is ISR — generated on first request and cached — so an edit
+    // to an already-published post would otherwise not appear until the
+    // revalidate window expired, up to an hour of a writer concluding that
+    // saving is broken. The index and feeds are force-dynamic and need no hook.
     expect(POSTS).toContain('afterChange');
     expect(POSTS).toContain('afterDelete');
-    expect(POSTS).toMatch(/revalidatePath\('\/blog'\)/);
     expect(POSTS).toMatch(/revalidatePath\(`\/blog\/\$\{[^}]+\}`\)/);
   });
 });