feat: resolve live Nostr references in blog posts and add embeds

Support naddr/nevent/note slugs for unindexed posts, cache naddr lookups,
render Nostr embeds in markdown, and add a consistency check for events mirrors.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/blog/[slug]/page.tsx

@@ -30,7 +30,9 @@ export async function generateMetadata({ params }: Props): Promise<Metadata> {
     post.excerpt ||
     `Read "${post.title}" on the Belgian Bitcoin Embassy blog.`;
   const author = post.authorName || "Belgian Bitcoin Embassy";
-  const ogImageUrl = `/og?title=${encodeURIComponent(post.title)}&type=blog`;
+  // Prefer the post's own header image (Nostr `image` tag); fall back to a
+  // generated OG card so every post still gets a real preview image.
+  const ogImageUrl = post.image || `/og?title=${encodeURIComponent(post.title)}&type=blog`;
 
   return {
     title: post.title,