fix: log source URL in link_extraction LLM call logs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/services/source_scraper.rs

@@ -228,7 +228,7 @@ pub async fn extract_article_links_with_llm(
         crate::db::llm_call_log::insert(
             pool, uid, jid, "link_extraction", model,
             &system, &user, &response_str, llm_duration as i32,
-            None,
+            Some(source_url),
         ).await.ok();
     }