fix: add theme_id to preferred sources pipeline test

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

backend/tests/pipeline_test.rs

@@ -634,7 +634,7 @@ async fn preferred_sources_processed_first() {
     assert!(status_b.is_success(), "Source B creation should succeed");
 
     // Mark source A as preferred
-    let pref_body = serde_json::json!({ "source_ids": [source_a_id] });
+    let pref_body = serde_json::json!({ "source_ids": [source_a_id], "theme_id": theme_id });
     let (pref_status, _) = app
         .put_with_session("/api/v1/sources/preferred", &pref_body, &session)
         .await;