POST /<id>/agent/update was @jwt_required() only, so a read-only viewer could
trigger agent binary replacement + service restart across the whole fleet —
inconsistent with every other state-changing server route, which is
@developer_required. Add @developer_required and a test proving a viewer gets
403 while a developer clears the gate.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
