fix: add command to release a retired learner's email for reuse

[rahulkanneri-2u]

Description

A retired learner's original email is permanently blocked from re-registration, because is_email_retired()
still finds a User row whose email matches the retired-hash of that address.

This adds release_retired_learner_email(user) (accounts/utils.py), which replaces a fully-retired user's
hashed email with a placeholder (retired_email_<user_id>@<RETIRED_EMAIL_DOMAIN>), clearing that match so the
original address becomes reusable. The UserRetirementStatus row and retirement history are left untouched.

Exposed via a new release_retired_user_email management command (--username or --user_id) for
Operator/Support use. No automatic trigger (waffle flag, API hook) is wired up so this is manual only, for now.

Supporting information

• LP-1177 (private)
• Proposal doc (private)

Testing instructions

1. Retire a test user to the COMPLETE state.
2. is_email_retired("learner@example.com") => True.
3. ./manage.py lms release_retired_user_email --username <retired_username>
4. is_email_retired("learner@example.com") => False; confirm re-registration succeeds.
5. pytest openedx/core/djangoapps/user_api/accounts/tests/test_utils.py openedx/core/djangoapps/user_api/management/tests/test_release_retired_user_email.py

Deadline

None.

Other information

None.