fix: enable article history in preferred sources test

article_history_days=0 disables "used" trace entries, so the test
found 0 entries. Changed to 90 to enable tracing.

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

backend/tests/pipeline_test.rs

@@ -590,12 +590,12 @@ async fn preferred_sources_processed_first() {
     let theme_id: uuid::Uuid = theme_resp["id"].as_str().unwrap().parse().unwrap();
 
     // Settings: batch_size=1, source_extraction_window=10 so both sources
-    // are processed in a single wave, article_history_days=0 to disable dedup
+    // are processed in a single wave, article_history_days=90 to enable tracing
     let settings = serde_json::json!({
         "max_articles_per_source": 10,
         "max_links_per_source": 8,
         "use_brave_search": false,
-        "article_history_days": 0,
+        "article_history_days": 90,
         "batch_size": 1,
         "source_extraction_window": 10,
         "search_agent_behavior": "",