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/swf/conversion-worker.ts

@@ -0,0 +1,55 @@
+import { parentPort } from "node:worker_threads";
+import { convertSwfToNitro, extractIconFromSwf } from "./index";
+
+export interface ConvertRequest {
+	id: number;
+	type: "convert";
+	buffer: Buffer;
+	classname: string;
+}
+
+export interface IconRequest {
+	id: number;
+	type: "icon";
+	buffer: Buffer;
+	classname: string;
+}
+
+export type WorkerRequest = ConvertRequest | IconRequest;
+
+export type WorkerResponse =
+	| {
+			id: number;
+			ok: true;
+			result?: import("./index").ConversionResult;
+			icon?: Buffer | null;
+	  }
+	| { id: number; ok: false; error: string };
+
+if (parentPort) {
+	parentPort.on("message", (msg: WorkerRequest) => {
+		try {
+			if (msg.type === "convert") {
+				const result = convertSwfToNitro(msg.buffer, msg.classname);
+				parentPort!.postMessage({
+					id: msg.id,
+					ok: true,
+					result,
+				} satisfies WorkerResponse);
+			} else {
+				const icon = extractIconFromSwf(msg.buffer, msg.classname);
+				parentPort!.postMessage({
+					id: msg.id,
+					ok: true,
+					icon: icon ?? null,
+				} satisfies WorkerResponse);
+			}
+		} catch (err) {
+			parentPort!.postMessage({
+				id: msg.id,
+				ok: false,
+				error: (err as Error).message,
+			} satisfies WorkerResponse);
+		}
+	});
+}