Add direct file download links to completed requests

Embeds a signed JWT download token (30-day expiry) in the requests API
response so users can download completed audiobook/ebook files directly
from the UI or by sharing the URL to apps like BookPlayer — no session
cookie required.

- jwt.ts: add generateDownloadToken / verifyDownloadToken helpers
- api/requests: append downloadUrl to completed requests with a filePath
- api/requests/[id]/download: new token-authenticated streaming endpoint;
  serves single files directly or zips multi-file audiobooks with adm-zip
- RequestCard: add Download link in the actions area for completed requests

src/components/requests/RequestCard.tsx

@@ -26,12 +26,15 @@ interface RequestCardProps {
     createdAt: string;
     updatedAt: string;
     completedAt?: string;
+    downloadUrl?: string | null;
     audiobook: {
       id: string;
       audibleAsin?: string;
       title: string;
       author: string;
       coverArtUrl?: string;
+      filePath?: string | null;
+      fileFormat?: string | null;
     };
   };
   showActions?: boolean;
@@ -49,6 +52,7 @@ export function RequestCard({ request, showActions = true }: RequestCardProps) {
   const requestType = request.type || 'audiobook';
   const isEbook = requestType === 'ebook';
 
+  const isCompleted = ['available', 'downloaded'].includes(request.status);
   const canCancel = ['pending', 'searching', 'downloading'].includes(request.status);
   const isActive = ['searching', 'downloading', 'processing'].includes(request.status);
   const isFailed = request.status === 'failed';
@@ -271,6 +275,18 @@ export function RequestCard({ request, showActions = true }: RequestCardProps) {
                     </Button>
                   </>
                 )}
+                {isCompleted && request.downloadUrl && (
+                  <a
+                    href={request.downloadUrl}
+                    className="inline-flex items-center gap-1 text-xs text-blue-600 dark:text-blue-400 hover:text-blue-700 dark:hover:text-blue-300 hover:underline"
+                  >
+                    <svg className="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                      <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2}
+                        d="M4 16v1a3 3 0 003 3h10a3 3 0 003-3v-1m-4-4l-4 4m0 0l-4-4m4 4V4" />
+                    </svg>
+                    Download
+                  </a>
+                )}
                 {canCancel && (
                   <Button
                     onClick={handleCancel}