fix: resolve actual article source instead of showing aggregator domain

Google News RSS feeds report every article as coming from
"news.google.com" because the source was extracted from the feed URL
rather than the individual article. This adds a multi-strategy
resolver that extracts the real publisher name via:

1. SimplePie <source> element (Google News provides this)
2. Title suffix parsing ("Headline - Source" format)
3. HTTP HEAD redirect resolution as a fallback

Also adds a one-click "Fix Article Sources" backfill button on the
settings page to correct existing articles in the database.

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

includes/class-peptide-news.php

@@ -80,6 +80,7 @@ class Peptide_News {
 
         $this->loader->add_action( 'peptide_news_cron_fetch', $fetcher, 'fetch_all_sources' );
         $this->loader->add_filter( 'cron_schedules', $fetcher, 'add_custom_cron_schedules' );
+        $this->loader->add_action( 'wp_ajax_peptide_news_backfill_sources', $fetcher, 'ajax_backfill_sources' );
     }
 
     /**