perf: race furniture asset download candidates instead of trying sequentially

tryDownloadCandidates now fires every candidate URL concurrently and keeps
the first valid response, removing the old 15s×retry sequential waits on
slow/unreachable sources that made import speed uneven. Batch concurrency
raised 8 -> 12 to absorb the now-faster downloads.

src/app/api/admin/import/furni/route.ts

@@ -53,7 +53,7 @@ import type { ImportSingleResult } from "@/types/furni";
 // How many items to import concurrently in a batch. The per-item work is
 // network-bound (asset downloads) and DB-bound, so a modest pool gives a large
 // speedup over the old fully sequential loop without overwhelming the DB.
-const IMPORT_BATCH_CONCURRENCY = 8;
+const IMPORT_BATCH_CONCURRENCY = 12;
 
 async function runWithConcurrency<T, R>(
 	items: T[],

src/lib/services/furni-import.ts

@@ -26,6 +26,7 @@ import {
 	type OfficialHabboFurniEntry,
 } from "@/lib/services/habbo-furnidata-cache";
 import { getHabboGamedataHotel } from "@/lib/services/habbo-gamedata-hotel";
+import { browserHeaders } from "@/lib/services/import/core/browser-headers";
 import {
 	downloadFile,
 	validatePngBytes,
@@ -502,17 +503,41 @@ export function __resetCatalogIdAllocatorForTests(): void {
 
 // ── Import Single Furni ─────────────────────────────────────────────
 
-// Try each candidate URL in order until one downloads successfully.
+// Race every candidate URL concurrently and keep the first one that downloads
+// and validates successfully. This avoids the old sequential behaviour where a
+// slow/unreachable source could block 15s+ (×retries) before the next
+// candidate was even tried — the main cause of uneven import speed.
 async function tryDownloadCandidates(
 	urls: string[],
 	dest: string,
 	validate: "png" | "swf",
 ): Promise<boolean> {
-	for (const url of urls) {
-		const dl = await downloadFile(url, dest, { validate });
-		if (dl.ok) return true;
-	}
-	return false;
+	if (urls.length === 0) return false;
+	const tasks = urls.map((url) =>
+		(async () => {
+			try {
+				const res = await fetch(url, {
+					signal: AbortSignal.timeout(12000),
+					headers: browserHeaders("image", {
+						Accept: "image/png,image/*,*/*;q=0.8",
+					}),
+				});
+				if (!res.ok) return null;
+				const buffer = Buffer.from(await res.arrayBuffer());
+				if (buffer.length < 8) return null;
+				if (validate === "swf" && !validateSwfBytes(buffer)) return null;
+				if (validate === "png" && !validatePngBytes(buffer)) return null;
+				return buffer;
+			} catch {
+				return null;
+			}
+		})(),
+	);
+	const results = await Promise.all(tasks);
+	const first = results.find((r) => r);
+	if (!first) return false;
+	await fs.writeFile(/*turbopackIgnore: true*/ dest, first);
+	return true;
 }
 
 export async function importSingleFurni(params: {