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/i18n/locales/en.json

@@ -88,7 +88,9 @@
       "spotsLeft": "spots left",
       "soldOut": "Sold Out",
       "cancelled": "Cancelled",
-      "eventEnded": "Event Ended"
+      "eventEnded": "Event Ended",
+      "registrationClosed": "Registration Closed",
+      "registrationClosesAt": "Registration closes {date}"
     },
     "booking": {
       "join": "Join Event",