Add in churned user win-back email

[vanZeben]

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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.

[andrewlimaza]

Thanks for contributing to the Paid Memberships Pro platform @vanZeben , this is a great feature that will need a bit more thought of how/where we implement this.

I didn't want to leave this uncommented as it's not gone unnoticed.

I am going to bring this to our development team in our next meeting to consider releasing in the future. It may be better suited in a separate Add On such as the Abandoned Cart or similar.

[dparker1005]

Thank you again for your contribution, @vanZeben.

I have marked this PR as a draft since, as @andrewlimaza mentioned, this functionality would likely be released as an Add On instead of included in the core PMPro plugin but we do want to preserve the work that you have put into this feature in case an Add On is pursued in the future.

In the meantime, for anyone looking for paths to send these kind of "churned user" emails, here are our current suggestions for the best way to accomplish this:
https://www.paidmembershipspro.com/re-engage-past-members-win-back/