articles/[slug]: drop Stay Updated sidebar card; related fallback to latest

Empty Related Articles bug on /articles/<slug> traced to two issues:
- Two rows were imported with category="industry" instead of the canonical
  "Industry News", so the category-match query returned zero peers.
- The fallback path was missing — getLegacyRelatedArticles now tops up
  with the latest published rows whenever the category match is short.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/articles/[slug]/ArticleDetailClient.tsx

@@ -424,17 +424,6 @@ export default function ArticleDetailClient({ article, relatedArticles }: Articl
               </div>
             </div>
 
-            {/* Newsletter Signup */}
-            <div className="bg-[#0d1520] border border-[#1e3a5f] p-5">
-              <h3 className="font-heading text-[#e0e0e0] font-bold mb-2">Stay Updated</h3>
-              <p className="font-body text-[#777] text-xs mb-4">
-                Get the latest broadcast engineering news delivered to your inbox.
-              </p>
-              <Link href="/home-page#newsletter" className="btn-subscribe text-xs py-2 px-4 inline-block w-full text-center">
-                Subscribe Free
-              </Link>
-            </div>
-
             {/* Sidebar ad stack — 300x600 + 300x250 rotating */}
             <SidebarAdStack />
           </aside>