Joost Boonzajer Flaes via Elementary: Add volume anomaly tests to staging models

[joostboon]

Summary\n\nAdds volume anomaly tests to stg_orders and stg_payments to improve upstream coverage for the critical cpa_and_roas model.\n\n### Changes\n- stg_orders: Added elementary.volume_anomalies test to detect sudden drops or spikes in row count that could silently reduce the number of orders flowing into downstream financial models.\n- stg_payments: Added elementary.volume_anomalies test to monitor payment volume, since payment records determine the amount column used in revenue and ROAS calculations.\n\n### Why\nBoth staging models feed into the critical cpa_and_roas model (via orders → customer_conversions → cpa_and_roas). Without volume monitoring, data ingestion failures could go undetected and corrupt CPA and ROAS metrics.

Created by: joost@elementary-data.com