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-activator.php

@@ -36,6 +36,7 @@ class Peptide_News_Activator {
             content LONGTEXT NOT NULL,
             author VARCHAR(255) NOT NULL DEFAULT '',
             thumbnail_url VARCHAR(2048) NOT NULL DEFAULT '',
+            thumbnail_local VARCHAR(512) NOT NULL DEFAULT '',
             published_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
             fetched_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
             categories VARCHAR(500) NOT NULL DEFAULT '',