Vela holds one ServerKit API key as a host-side secret and polls the
fleet list, so this read route moves from bare @jwt_required() to
auth_required() + require_scope('servers:read') — a deliberate,
per-route jwt_only_census exception, not a sweep. JWT callers are
unchanged; a restricted key without servers:read gets 403 from the
scope middleware, no credentials get 401.
Scope choice: the existing servers:read catalog entry already covers
'list servers, agents, and metrics', and the serverkit-gui read routes
(capabilities/frame/surface, sibling repo) take the same scope, so one
key scope serves the whole Vela polling surface.
The census ceiling ratchets down to 600 and the migration inventory is
regenerated, as both ratchets require. tests/test_servers_api_key_scope.py
pins the four cases: scoped key 200, wrong-scope key 403, JWT 200,
anonymous 401.
