All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR adds a new "Churned Member Win-Back Email" feature that sends an email to former members a configurable number of days after their membership expires. This helps site owners re-engage with former members and potentially win them back.

Key changes:

• Added a new email template membership_churned that gets sent to former members
• Added a setting in the Email Settings page to configure the number of days after expiration (default 30)
• Created a daily cron job that finds members whose memberships expired exactly N days ago
• Only sends the email once per expired membership (tracked via user meta)

How to test the changes in this Pull Request:

1. Set up a membership site with at least one member
2. Let that member's membership expire (or manually set the status to 'expired' in the database)
3. Change the pmpro_churned_email_days setting to a smaller value for testing (e.g., 1 day)
4. Wait for the daily cron to run, or manually trigger it for testing
5. Verify the member receives the "We miss you" email

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Added a new "Churned Member Win-Back Email" feature that sends an email to former members a configurable number of days after their membership expires. Site owners can customize the email content and the number of days after expiration before sending.