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/collections/Posts.ts

@@ -4,21 +4,22 @@ import { lexicalEditor, BlocksFeature } from '@payloadcms/richtext-lexical';
 import { Callout } from '@/blocks/Callout';
 
 /**
- * Revalidate everything a post appears on.
+ * Drop the cached copy of a post page when it changes.
  *
- * Blog pages are ISR: CI builds the image with no database reachable, so they
- * cannot be statically generated at build time. Without this, a published post
- * would not appear until the revalidate window expired — up to an hour of a
- * writer reasonably concluding that publishing is broken.
+ * Only the post page needs this. The blog index, the RSS feed and the content
+ * sitemap are force-dynamic — they have no dynamic params, so Next would
+ * prerender them at build time, where CI has no database — which means they
+ * already reflect a change on the next request.
+ *
+ * /blog/[slug] is ISR: generated on first request and cached, so without this
+ * an edit to an already-published post would not appear until the revalidate
+ * window expired — up to an hour of a writer concluding that saving is broken.
  *
  * Payload runs in the same process as Next, so this is a direct revalidatePath
  * call: no webhook, no shared secret, no network hop to get wrong.
  */
 function revalidatePost(slug: string) {
-  revalidatePath('/blog');
   revalidatePath(`/blog/${slug}`);
-  revalidatePath('/content-sitemap.xml');
-  revalidatePath('/blog/rss.xml');
 }
 
 export const Posts: CollectionConfig = {