Hide draft articles from the front page and mark drafts when viewed directly

Add an Article::published() scope and use it on the front page (which
previously showed drafts), the /blog listing and the feeds. Draft
articles are still reachable at their URL, but now show a draft notice,
a "[Draft]" title prefix and a noindex robots meta tag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

resources/views/master.blade.php

@@ -22,6 +22,7 @@
         @if (File::exists(public_path('gpg.key')))
             <link rel="pgpkey" href="{{ config('app.url')}}/gpg.key">
         @endif
+        @stack('head')
     </head>
     <body>
         <header>