Only SameSite=Lax protected the acknowledge and zone save/delete endpoints,
even though a Csrf helper and mintable token already existed. Add
GET /api/csrf (CsrfController::token) to mint/return a per-session token,
check it (auth -> rbac -> csrf -> mutate) in EventDetailController::acknowledge
and ZoneConfigController::save/delete, and wire app.js/zone-draw.js to fetch
the token on load and send it as X-Csrf-Token on every POST.
