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/epub/fixtures.rs

@@ -35,6 +35,7 @@ pub fn article(id: ArticleId, entry_id: EntryId, title: &str) -> Article {
         first_seen: timestamp(),
         url: format!("https://example.com/{entry_id}"),
         author: Some("A. Writer".into()),
+        publication: None,
         feed_id: 7,
         feed_title: "Example Feed".into(),
         category: Some("Tech".into()),