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-rest-api.php

@@ -134,7 +134,7 @@ class Peptide_News_Rest_API {
 
         $articles = $wpdb->get_results( $wpdb->prepare(
             "SELECT id, source, source_url, title, excerpt, author, thumbnail_url,
-                    published_at, categories, tags
+                    thumbnail_local, published_at, categories, tags
              FROM {$table}
              WHERE is_active = 1
              ORDER BY published_at DESC