fix: expect 201 Created for source creation in syntheses test

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

backend/tests/api_syntheses_test.rs

@@ -667,7 +667,7 @@ async fn generate_pipeline_resolves_model_from_admin_config() {
     let (source_status, _) = app
         .post_with_session("/api/v1/sources", &source_body, &session)
         .await;
-    assert_eq!(source_status, StatusCode::OK, "Source creation should succeed");
+    assert_eq!(source_status, StatusCode::CREATED, "Source creation should succeed");
 
     // Trigger generation — this will run async. The key assertion is that
     // the HTTP trigger returns 202 (not 500) and the async job doesn't crash