* fix(proxy): gate team allowed_passthrough_routes to proxy admins
allowed_passthrough_routes short-circuits the role-based route gate, so
the keys endpoints already restrict it to proxy admins. The team writers
(/team/new, /team/update) had no equivalent check, letting an org admin
(a non-proxy-admin who clears the route gate and _verify_team_access)
self-grant pass-through routes on their team. Lift the keys check into a
shared helper and apply it to both team endpoints.
Resolves LIT-3019
* docs(proxy): note view-only admins are intentionally excluded from passthrough gate
Clarifies the proxy-admin guard per review feedback; no behavior change.
Refs LIT-3019