Replace broken DDG search with AI thumbnail generation fallback

DuckDuckGo's HTML search endpoint now requires vqd session tokens,
making server-side search-based URL resolution unreliable. Simplified
the thumbnail pipeline:

- OG scrape only for non-aggregator URLs (direct publisher pages)
- AI image generation via OpenRouter for aggregator articles (Google News)
- Added Image Model settings field in admin UI
- Added thumbnail_local column for AI-generated images
- Updated public/admin display to check thumbnail_local as fallback
- Removed all DuckDuckGo search code (find_article_url_via_search,
  extract_ddg_url, resolve_article_for_scraping)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/class-peptide-news-llm.php

@@ -390,12 +390,12 @@ class Peptide_News_LLM {
         global $wpdb;
         $table = $wpdb->prefix . 'peptide_news_articles';
 
-        // Find active articles with no thumbnail (neither URL nor local).
+        // Find active articles with no usable thumbnail (including _no_image sentinel).
         $articles = $wpdb->get_results( $wpdb->prepare(
             "SELECT id, title, excerpt, ai_summary, tags, categories
              FROM {$table}
              WHERE is_active = 1
-               AND ( thumbnail_url = '' OR thumbnail_url IS NULL )
+               AND ( thumbnail_url = '' OR thumbnail_url IS NULL OR thumbnail_url = '_no_image' )
                AND ( thumbnail_local = '' OR thumbnail_local IS NULL )
              ORDER BY fetched_at DESC
              LIMIT %d",