Events gain nullable presale_closure_enabled / presale_close_minutes_before
overrides; null inherits the new site_settings defaults (enabled, 120 min).
A shared resolver computes the effective cutoff, which the public event API
exposes as presaleClosesAt and the public booking endpoint enforces. Door and
admin ticket creation are not gated.
Admin: toggle + duration picker in the event modal (pre-filled from the site
default) and a matching default card on Settings › General. Public: the event
page shows "Registration Closed" after the cutoff and the checkout page
redirects back.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>