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>

app/Models/Article.php

@@ -101,6 +101,14 @@ class Article extends Model
         );
     }
 
+    /**
+     * Scope a query to only include published (non-draft) articles.
+     */
+    public function scopePublished(Builder $query): Builder
+    {
+        return $query->where('published', 1);
+    }
+
     /**
      * Scope a query to only include articles from a particular year/month.
      */