perf: offload SWF→Nitro conversion to a worker-thread pool

The synchronous convertSwfToNitro / extractIconFromSwf calls (pure CPU,
no DB/network) now run in a small worker pool so the main Node event
loop stays free during imports — the dominant import-time CPU spike is
moved off the request thread. The pool degrades gracefully to a
synchronous fallback if workers can't be created, and is skipped in the
test environment.

src/lib/services/furni-import.test.ts

@@ -24,10 +24,20 @@ vi.mock("@/lib/services/import/core/download", () => ({
 	validateSwfBytes: vi.fn(),
 }));
 
-vi.mock("@/lib/services/swf-to-nitro", () => ({
-	convertSwfToNitro: vi.fn(),
-	extractIconFromSwf: vi.fn(),
-}));
+vi.mock("@/lib/services/swf-to-nitro", () => {
+	const convertSwfToNitro = vi.fn();
+	const extractIconFromSwf = vi.fn();
+	return {
+		convertSwfToNitro,
+		extractIconFromSwf,
+		convertSwfToNitroAsync: vi.fn(async (buffer: Buffer, classname: string) =>
+			convertSwfToNitro(buffer, classname),
+		),
+		extractIconFromSwfAsync: vi.fn(async (buffer: Buffer, classname: string) =>
+			extractIconFromSwf(buffer, classname),
+		),
+	};
+});
 
 import {
 	autoPriceFurni,