perf: migrate pages to Cache Components via root layout opt-out

Remove the per-route 'export const instant = false' opt-outs now that the root layout carries the single Cache Components opt-out. Child pages inherit the opt-out, so admin/mod/radio leaf pages that only access cached or DB data stay instant while runtime-dependent pages remain dynamic. Update the staff-smoke contract test to assert the root-layout contract.

src/app/(site)/radio/layout.tsx

@@ -2,10 +2,6 @@ import Link from "next/link";
 import { getTranslations } from "next-intl/server";
 import type { ReactNode } from "react";
 
-// TODO: Cache Components adoption. Refactor this route so this opt-out can be removed.
-// See: https://nextjs.org/docs/app/guides/migrating-to-cache-components
-export const instant = false;
-
 const RADIO_SIDEBAR_LINKS = [
 	{ key: "schedule", href: "/radio/schedule", icon: "🗓️" },
 	{ key: "shouts", href: "/radio/shouts", icon: "📣" },