Feature Request: Add Bounce Rate to Jetpack Stats for Subscriptions Newsletter

### Impacted plugin

Jetpack

### What

Add email bounce rate tracking to the Jetpack Stats dashboard for Jetpack Subscriptions.

### How

Currently, Jetpack provides stats on email opens and clicks, but there’s no tracking for soft or hard bounces.

Many email marketing tools track these. Some clean bounced addresses automatically, while others share a list of emails and let the user prune their list.

Jetpack could enhance its analytics by displaying bounce rates, and for email subscribers, display the bounced email addresses, so users can retain or manage them as needed.

Start by tracing the Jetpack Stats dashboard and Jetpack Subscriptions entry points, then compare how existing email-open and click metrics are collected and displayed. Done means the requested soft- and hard-bounce rates and bounced-address list are defined and shown, with the intended subscriber-management behavior clarified.

Written by the indexing model from the issue text.