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/import-live.test.ts

@@ -43,8 +43,8 @@ vi.mock("@/lib/services/import/core/download", async () => {
 	};
 });
 
-vi.mock("@/lib/services/swf-to-nitro", () => ({
-	convertSwfToNitro: vi.fn(() => ({
+vi.mock("@/lib/services/swf-to-nitro", () => {
+	const convertSwfToNitro = vi.fn((_buffer: Buffer, _classname: string) => ({
 		bundle: Buffer.alloc(256, 0x42),
 		classname: "n/a",
 		dimensions: { x: 1, y: 1, z: 1 },
@@ -61,10 +61,23 @@ vi.mock("@/lib/services/swf-to-nitro", () => ({
 			defaultdir: 0,
 			partcolors: { color: [] },
 		},
-	})),
-	extractIconFromSwf: vi.fn(() => Buffer.from("fake-icon")),
-	parseNitroBundle: vi.fn(() => ({ json: null })),
-}));
+	}));
+	const extractIconFromSwf = vi.fn((_buffer: Buffer, _classname: string) =>
+		Buffer.from("fake-icon"),
+	);
+	const parseNitroBundle = vi.fn(() => ({ json: null }));
+	return {
+		convertSwfToNitro,
+		extractIconFromSwf,
+		parseNitroBundle,
+		convertSwfToNitroAsync: vi.fn(async (buffer: Buffer, classname: string) =>
+			convertSwfToNitro(buffer, classname),
+		),
+		extractIconFromSwfAsync: vi.fn(async (buffer: Buffer, classname: string) =>
+			extractIconFromSwf(buffer, classname),
+		),
+	};
+});
 
 vi.mock("@/lib/services/habbo-furnidata-cache", () => ({
 	lookupOfficialHabboFurni: vi.fn(async () => null),