Add manual-import and download-access features

Introduce manual import workflow and download permission support. Adds a Prisma migration and schema field (users.download_access) to track per-user download access, and updates admin UI to toggle global and per-user download access. Implements new APIs: filesystem browse, manual-import endpoint, download-access settings, audiobook download-status, and on-demand download-token generation. Adds frontend components for manual import and related tests, plus documentation for the manual-import feature and the documentation-agent prompt. Key files: prisma/migrations/20260212000000_add_download_access_permission/migration.sql, prisma/schema.prisma, src/app/api/admin/filesystem/browse/route.ts, src/app/api/admin/manual-import/route.ts, src/app/api/admin/settings/download-access/route.ts, src/app/api/requests/[id]/download-token/route.ts, src/app/api/audiobooks/[asin]/download-status/route.ts, and updated admin users pages/components and permissions util.

src/components/requests/RequestCard.tsx

@@ -27,7 +27,7 @@ interface RequestCardProps {
     createdAt: string;
     updatedAt: string;
     completedAt?: string;
-    downloadUrl?: string | null;
+    downloadAvailable?: boolean;
     audiobook: {
       id: string;
       audibleAsin?: string;
@@ -276,18 +276,6 @@ 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}