Achievements: Change show_on_profile default to false
Part of the agreed Engineering Implementation Tasks from the [February 2026 GA roadmap](https://gitlab.com/groups/gitlab-org/-/work_items/9429#note_3101433934):
> All users of the GitLab instance can be awarded badges, but an explicit approve action is required to accept an awarded badge. Continues the effort started in [#390389](https://gitlab.com/gitlab-org/gitlab/-/issues/390389).
## What
Single migration: change the DB default of `show_on_profile` on `achievements_user_achievements` from `true` to `false` so awarded badges do not appear on a user's profile until explicitly accepted.
## Notes
- The `show_on_profile` field already exists (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/161225). this is a default value change only
- Prerequisite for the approve/decline UI issue
issue