bug: billing alert recipients aren't removed when a user leaves an organization

[isshaddad]

Provide environment information

n/a

Describe the bug

When a user leaves (or is removed from) an organization, their email address is not removed from the org's billing alert recipients. They continue receiving billing alert emails even though they no longer belong to the organization.

Expected: removing/leaving a user from an org should also remove their email from any billing alert recipient list tied to that org.

Reproduction repo

n/a

To reproduce

1. As an org admin, add a user to the organization.
2. Add that user's email as a billing alert recipient.
3. Remove the user from the org (or have them leave).
4. The removed user is still on the billing alerts recipient list

Additional information

n/a

[sajdakabir]

would love to work on this issue