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/programs/[slug].astro

@@ -3,6 +3,7 @@ import Base from '../../layouts/Base.astro';
 import PageHero from '../../components/PageHero.astro';
 import { programs } from '../../data/programs';
 import { performanceClassPricing } from '../../data/pricing';
+import { CATEGORIES } from '../../lib/blog-const';
 export const prerender = true;
 
 export function getStaticPaths() {
@@ -10,7 +11,6 @@ export function getStaticPaths() {
 }
 const { program } = Astro.props;
 const others = programs.filter((p) => p.slug !== program.slug);
-const blogCategories = ['Fitness', 'Salute', 'Stile di vita', 'Nutrizione'];
 ---
 <Base title={program.title} description={program.excerpt}>
   <PageHero title={program.title.toUpperCase()} />
@@ -34,7 +34,7 @@ const blogCategories = ['Fitness', 'Salute', 'Stile di vita', 'Nutrizione'];
       </div>
       <aside class="prog__side">
         <h3>Categorie</h3>
-        {blogCategories.map((c) => <p><a href="/blog">{c}</a></p>)}
+        {CATEGORIES.map((c) => <p><a href={`/blog?categoria=${encodeURIComponent(c)}`}>{c}</a></p>)}
         <h3>Altri programmi</h3>
         {others.map((o) => <p><a href={`/programs/${o.slug}`}>{o.title}</a></p>)}
       </aside>