fix: sitemap senza admin, categorie sidebar reali, 404 blog, og:image e README

- IMP-1: sitemap filter esclude /admin
- IMP-2: [slug].astro programmi usa CATEGORIES da blog-const con link /blog?categoria=
- MIN-1: blog/[slug].astro ritorna 404 vera invece di redirect
- MIN-3: public/img/og.jpg + meta og:image in Base.astro
- README: porta 3000→4321, Astro 5→7

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/pages/blog/[slug].astro

@@ -6,7 +6,7 @@ import { getPublishedBySlug } from '../../lib/posts';
 export const prerender = false;
 
 const post = getPublishedBySlug(getDb(), Astro.params.slug!);
-if (!post) return Astro.redirect('/404');
+if (!post) return new Response(null, { status: 404 });
 const fmt = new Intl.DateTimeFormat('it-IT', { dateStyle: 'long' });
 ---
 <Base title={post.title} description={post.excerpt}>