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-correlation.test.ts

@@ -32,6 +32,7 @@ vi.mock("@/lib/services/catalog-git-queue", () => ({
 	beginCatalogExport: vi.fn(),
 }));
 
+import { emptyRouteContext } from "@/test/route-context";
 import { withAdmin } from "./api-handler";
 
 const request = () =>
@@ -50,7 +51,10 @@ describe("admin API correlation", () => {
 			await Promise.resolve();
 			return NextResponse.json(getOperationContext());
 		});
-		const [a, b] = await Promise.all([handler(request()), handler(request())]);
+		const [a, b] = await Promise.all([
+			handler(request(), emptyRouteContext()),
+			handler(request(), emptyRouteContext()),
+		]);
 		expect(a.headers.get("x-operation-id")).not.toBe("forged-client-id");
 		expect(a.headers.get("x-operation-id")).not.toBe(
 			b.headers.get("x-operation-id"),
@@ -71,7 +75,7 @@ describe("admin API correlation", () => {
 			response.cookies.set("second", "two", { httpOnly: true });
 			return response;
 		});
-		const response = await handler(request());
+		const response = await handler(request(), emptyRouteContext());
 		expect(response.status).toBe(307);
 		expect(response.headers.get("location")).toBe(
 			"https://hotel.test/admin/users",
@@ -104,7 +108,7 @@ describe("admin API correlation", () => {
 					},
 				),
 		);
-		const response = await handler(request());
+		const response = await handler(request(), emptyRouteContext());
 		expect(response.headers.get("content-type")).toBe("text/event-stream");
 		expect(response.headers.get("x-custom")).toBe("retained");
 		const body = response.text();
@@ -121,17 +125,17 @@ describe("admin API correlation", () => {
 			throw new Error("private-database-message");
 		});
 		state.authorized = false;
-		const unauthorized = await handler(request());
+		const unauthorized = await handler(request(), emptyRouteContext());
 		expect(unauthorized.status).toBe(401);
 		expect(state.record).not.toHaveBeenCalled();
 		expect(unauthorized.headers.get("x-operation-id")).toBeTruthy();
 		state.authorized = true;
 		state.permitted = false;
-		const forbidden = await handler(request());
+		const forbidden = await handler(request(), emptyRouteContext());
 		expect(forbidden.status).toBe(403);
 		expect(forbidden.headers.get("x-operation-id")).toBeTruthy();
 		state.permitted = true;
-		const failed = await handler(request());
+		const failed = await handler(request(), emptyRouteContext());
 		expect(failed.status).toBe(500);
 		expect(failed.headers.get("x-operation-id")).toBeTruthy();
 		expect(await failed.json()).toEqual({