PUT /admin/users/:id validated password and role but not username, even
though updateUser writes req.body.username. A blank/too-short username
could be saved, and a duplicate hit the DB unique constraint and
surfaced as an opaque 500.
- Route: add the same validator used on create,
body('username').optional().isString().trim().isLength({min:3,max:32}).
The trim sanitizer also collapses whitespace-only input so it fails
the min-length check.
- Controller: when the username is changing, pre-check for another user
with that name and return 409 instead of letting the DB throw a 500.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
