perf: speed up batch furniture import

- Run batch imports with bounded concurrency (IMPORT_BATCH_CONCURRENCY = 8)
  instead of fully sequentially, parallelizing network downloads and DB writes.
- Collect FurnitureData.json entries and write the whole file once per batch
  (appendFurniEntriesBulk) instead of rewriting it on every single item.

Catalog id allocation stays serialized and FurnitureData writes remain
lock-guarded, so concurrent imports are safe.

src/lib/services/furni-data.ts

@@ -317,6 +317,37 @@ export async function appendFurniEntry(
 	});
 }
 
+/**
+ * Append many FurnitureData entries in a single read/parse/write cycle
+ * (under the furnidata lock). Used by batch imports so the whole JSON file
+ * is rewritten once instead of once per item.
+ */
+export async function appendFurniEntriesBulk(
+	entries: Array<{ entry: Record<string, unknown>; itemType: string }>,
+): Promise<void> {
+	if (entries.length === 0) return;
+	await withFurniDataLock(async () => {
+		const furniData = (await readFurniData()) as Record<
+			string,
+			{ furnitype: Array<Record<string, unknown>> }
+		>;
+		const existingOwners = furniDataIdOwners(furniData);
+		assertNoFurniDataIdConflicts(existingOwners, entries);
+		for (const { entry, itemType } of entries) {
+			if (itemType === "i") {
+				if (!furniData.wallitemtypes)
+					furniData.wallitemtypes = { furnitype: [] };
+				upsertEntryInSection(furniData.wallitemtypes, entry);
+			} else {
+				if (!furniData.roomitemtypes)
+					furniData.roomitemtypes = { furnitype: [] };
+				upsertEntryInSection(furniData.roomitemtypes, entry);
+			}
+		}
+		await writeFurniData(furniData as Record<string, unknown>);
+	});
+}
+
 export async function removeFurniEntry(classname: string): Promise<boolean> {
 	return withFurniDataLock(async () => {
 		const furniData = (await readFurniData()) as Record<