Three new POST endpoints on the existing admin blueprint, all guarded
by @require_admin and CSRF (per the global before_request hook):
/admin/users/<id>/deactivate flips active=False (refuses self)
/admin/users/<id>/activate flips active=True
/admin/users/<id>/delete hard delete with safeties:
- refuses self-delete
- refuses delete-of-the-last-admin
- refuses if the user owns Servers, Blueprints, or custom
Overlays (operator deletes those first via existing UIs)
- nulls out Job.user_id (jobs stay as audit trail; FK is nullable)
admin_users.html grows an Active column + an Actions column with the
appropriate button per row (none for self, Deactivate/Activate
toggle, Delete-with-confirmation modal). Modal pattern mirrors
blueprint_detail.html (same modal-close/modal-open data attrs,
csrf_token hidden field).
Refusal responses are 409 with a plain-text body (matches the
blueprint-in-use refusal at blueprint_routes.py:182). No flash
infrastructure introduced; consistent with the rest of the codebase.
All 367 existing tests still pass.
