Models are missing PII annotations, and ModerationAuditLog isn't retired #301

The openedx-forum 0.6.0 upgrade in openedx-platform is blocked because the new ModerationAuditLog model has no PII annotation, and make pii_check in openedx-platform fails on it. We're holding forum back to 0.5.x in the platform until this is fixed.

This repo's own PII check doesn't pass. Running the make test-pii command against v0.6.0:

Found 19 total models.
19 were eligible for annotation, 6 were annotated.
Coverage is 31.6%

None of the 13 forum.* models are annotated here. Up to now the platform has been carrying their annotations in its .annotation_safe_list.yml, so each new forum model breaks the platform's requirements upgrade instead of breaking CI in this repo. The check is never run here because pii_check isn't in the ci.yml matrix, and the pii_check tox env also fails on its own because it calls make without allowlist_externals.

ModerationAuditLog isn't cleared on retirement. It stores the moderated post's body and an original_author FK, but retire_all_content in src/forum/backends/mysql/api.py only scrubs Comment and CommentThread. A retired learner's post text stays in the audit log. This needs to be fixed before the model can be annotated with pii_retirement: local_api.

What needs to happen:

• Add PII annotations to the docstrings of all the models in src/forum/backends/mysql/models.py.
• Retire ModerationAuditLog rows in retire_all_content, either by scrubbing body or deleting them.
• Fix the pii_check tox env and add it to the CI matrix.
• Cut a release, then openedx-platform can drop its forum.* safe-list entries and unpin forum.