feat: add task workflow and asset downloads

app/api/assets/[id]/download/route.ts

@@ -0,0 +1,42 @@
+import { getAsset } from "@/lib/server/data-store";
+import { jsonError } from "@/lib/server/api";
+import { readAssetForDownload } from "@/lib/server/storage";
+
+export const runtime = "nodejs";
+
+export async function GET(_request: Request, context: { params: Promise<{ id: string }> }) {
+  try {
+    const { id } = await context.params;
+    const asset = await getAsset(id);
+    if (!asset) return jsonError("资产不存在", 404);
+    const file = await readAssetForDownload(asset);
+    if (!file) return jsonError("资产文件不可下载", 404);
+    return new Response(new Uint8Array(file.bytes), {
+      headers: {
+        "Content-Type": file.contentType,
+        "Content-Length": String(file.bytes.length),
+        "Content-Disposition": contentDisposition(asset.name || `${asset.id}${extensionForContentType(file.contentType)}`),
+        "Cache-Control": "private, no-store"
+      }
+    });
+  } catch (error) {
+    return jsonError(error, 500);
+  }
+}
+
+function contentDisposition(fileName: string): string {
+  const clean = fileName.replace(/[\r\n/\\]/g, "_").trim() || "download";
+  const ascii = clean.replace(/[^\x20-\x7E]/g, "_").replace(/"/g, "_");
+  return `attachment; filename="${ascii}"; filename*=UTF-8''${encodeURIComponent(clean)}`;
+}
+
+function extensionForContentType(contentType: string): string {
+  const normalized = contentType.split(";")[0]?.trim().toLowerCase();
+  if (normalized === "image/png") return ".png";
+  if (normalized === "image/jpeg") return ".jpg";
+  if (normalized === "image/webp") return ".webp";
+  if (normalized === "image/svg+xml") return ".svg";
+  if (normalized === "video/mp4") return ".mp4";
+  if (normalized === "audio/mpeg") return ".mp3";
+  return "";
+}