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[],