Keep the publication an article's page declares

Readability already reads og:site_name and JSON-LD publisher.name; carry
that through Page and Extracted and store it in articles.publication, the
same way the page byline is kept. `publication_label` is what the readers
will see after the feed name: the site name, else the domain, and nothing
when it would only repeat the feed's own title.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YWmCpUojfHXhSZ2129Z7Nv

src/social/mod.rs

@@ -328,6 +328,7 @@ mod tests {
             first_seen: ts("2026-08-15T05:00:00Z"),
             url: url.into(),
             author: None,
+            publication: None,
             feed_id: 1,
             feed_title: "Feed".into(),
             category: None,