Close pre-sale registration a configurable time before events start.

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>

frontend/src/app/(public)/events/[id]/page.tsx

@@ -25,6 +25,7 @@ interface Event {
   bannerUrl?: string;
   availableSeats?: number;
   bookedCount?: number;
+  presaleClosesAt?: string | null;
   createdAt: string;
   updatedAt: string;
 }
@@ -125,6 +126,7 @@ function generateEventJsonLd(event: Event) {
         : 'https://schema.org/SoldOut',
       url: `${siteUrl}/events/${event.slug}`,
       validFrom: new Date().toISOString(),
+      ...(event.presaleClosesAt ? { validThrough: event.presaleClosesAt } : {}),
     },
     image: event.bannerUrl
       ? (event.bannerUrl.startsWith('http') ? event.bannerUrl : `${siteUrl}${event.bannerUrl}`)