fix(build): make the production build pass, and stop it eating 20GB

`next build` had never completed on this host, so three real defects were
sitting in the tree untested. All three are now fixed and the build is green.

- The build was not memory-bound the way it looked. Turbopack's builder reached
  20.5GB RSS and died, and raising `--max-old-space-size` could never have
  helped: that flag caps the V8 heap, while the 20GB sat in Turbopack's own Rust
  allocator. The first symptom was misleading because the process doing the
  allocating is a grandchild of `npx`, so watching the direct child shows a
  95MB shim the whole time. Building with `--webpack` puts the build back under
  the JS heap, where the flag actually applies: peak 5.9GB, 150s, exit 0.

- withAdmin's second parameter was typed `{ params?: ... }` and given a `= {}`
  default, which made it optional and `RouteContext | undefined`. Next's
  generated route types assert that argument against `ParamCheck<RouteContext>`
  and reject it, across 113 route files. `tsc --noEmit` cannot see this, because
  Next only adds `.next/types` to the project during a production build — so the
  type check that everyone runs locally was structurally incapable of catching
  the only type error that blocks a deploy. `params` is now required, which is
  also what the code already assumed: it is awaited with no guard. The 35 test
  call sites that invoked a handler with one argument now pass a real context,
  and the await got a guard so a direct internal call cannot turn a missing
  context into a 500.

- `src/app/api/admin/import/furni/route.ts` re-exported `ensureDirectories` and
  `importSingleFurni` for "backward compatibility" that nothing used; the batch
  route imports from `@/lib/services/furni-import` directly. Next rejects any
  value export from a route module that is not an HTTP verb or config, so this
  had been breaking the build for as long as it existed. Removed.

- `isomorphic-dompurify` builds its server-side DOM through jsdom. Bundled, that
  pulls jsdom's `browser/default-stylesheet.css` into the server chunk, where the
  path no longer resolves, and page-data collection dies with ENOENT on every
  page that sanitizes HTML. Marked external so Node resolves it from
  node_modules and the standalone tracer includes it.

The remaining build warning is a pre-existing circular dependency between
chunks that share the webpack runtime. It costs hash reuse, not correctness, and
is left alone rather than churned here.

Verified: build exit 0, 276 static pages generated, 3223 tests pass, tsc and
biome clean.

src/lib/api-handler.ts

@@ -25,7 +25,11 @@ const MUTATING_METHODS = new Set(["POST", "PUT", "PATCH", "DELETE"]);
 const MAX_BODY_BYTES = 10 * 1024 * 1024; // 10 MB
 
 type AdminContext = NonNullable<Awaited<ReturnType<typeof getApiAdminContext>>>;
-type RouteContext = { params?: Promise<Record<string, string | string[]>> };
+// Next 15+ passes `params` as a promise and its generated route types require it
+// to be present, so this is deliberately not optional. `routeContext.params` is
+// awaited below without a guard, so an optional type would have been a lie
+// anyway. A route with no dynamic segments still receives `Promise<{}>`.
+type RouteContext = { params: Promise<Record<string, string | string[]>> };
 type AdminHandler = (
 	request: NextRequest,
 	context: AdminContext,
@@ -40,7 +44,16 @@ export function withAdmin(
 	},
 	handler: AdminHandler,
 ) {
-	return async (request: NextRequest, routeContext: RouteContext = {}) => {
+	// No default value here on purpose. `RouteContext` already has an optional
+	// `params`, so `{}` was always assignable and the default only widened the
+	// parameter to `RouteContext | undefined`. Next's generated route types assert
+	// the second argument against `ParamCheck<RouteContext>`, and the `undefined`
+	// fails that check. `tsc --noEmit` does not see it, because Next only adds
+	// `.next/types` to the project during a production build.
+	// Deliberately no default. A parameter with a default is optional in the
+	// signature, which makes its type `RouteContext | undefined` and fails Next's
+	// `ParamCheck`. Tests that call a handler directly pass `emptyRouteContext()`.
+	return async (request: NextRequest, routeContext: RouteContext) => {
 		const store = createStore("unknown" as IpAddress);
 		const { value: response, metrics } = await collectPerformance(() =>
 			runWithStore(store, async () => {
@@ -153,7 +166,11 @@ export function withAdmin(
 				operationId: store.requestId,
 				route: routeTemplate(
 					request.nextUrl.pathname,
-					await routeContext.params,
+					// Next always passes a context, and the type requires it, so
+					// this is only a guard against a direct internal call. The
+					// worst case is a slightly less specific metric label, which
+					// is not worth a 500.
+					await routeContext?.params,
 				),
 				method: request.method,
 				status: response.status,