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/admin/items/[id]/page.tsx

@@ -7,10 +7,6 @@ import { db, ItemsBase } from "@/lib/db";
 import { canAccess, getAdminContext, PERMS } from "@/lib/permissions";
 import { ItemsBaseEditForm } from "./items-base-edit-form";
 
-// 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;
-
 export default async function ItemsBaseDetailPage({
 	params,
 }: {

src/app/admin/items/page.tsx

@@ -7,10 +7,6 @@ import { db, ItemsBase } from "@/lib/db";
 import { canAccess, PERMS } from "@/lib/permissions";
 import { type ItemsBaseRow, ItemsTable } from "./items-table";
 
-// 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;
-
 export default async function ItemsBasePage({
 	params: paramsPromise,
 	searchParams,