Sender Health Summary

Returns per-sender health metrics for a team: sends, bounce rate, and at-risk flags. Bounce rate and bounces count invalid-recipient (list-quality) bounces only — aligned with in-app sender health score and sender list bounce rate. Field all_bounces (per sender) and total_all_bounces (summary) include every bounced send (soft, policy, etc.) for context. Use for "Which senders are at risk?"