Add admin Bulk Import feature

Introduce a Bulk Import feature for admins to scan server folders, match discovered audiobook folders against Audible, review matches, and queue batch imports.

What changed:
- Added documentation: documentation/features/bulk-import.md and TABLEOFCONTENTS update.
- Backend: SSE scan endpoint (POST /api/admin/bulk-import/scan) streams discovery and matching events; execute endpoint (POST /api/admin/bulk-import/execute) validates paths, creates/resolves audiobook & request records, and queues organize_files jobs. Both endpoints enforce admin-only access and validate allowed root directories (download_dir, media_dir, /bookdrop).
- Frontend: Modal wizard and steps for folder selection, scan progress, and match review (BulkImportWizard + ScanFolderStep, ScanProgressStep, MatchReviewStep + shared types).
- Utilities: bulk-import-scanner for folder discovery and ffprobe metadata extraction; shared types for scanned books/events.
- UI: Added Bulk Import quick action to admin dashboard (src/app/admin/page.tsx).

Key details:
- Audible searches are rate-limited (≈1.5s) and matching results include library/request status checks.
- Reuses existing organize_files job queue and manual-import pipeline; no new database tables introduced (state is ephemeral during the wizard).
- Includes error handling, path normalization, and security checks for allowed directories.

This commit wires frontend, backend, and docs together to provide an admin-only multi-step bulk import workflow.

src/app/admin/page.tsx

@@ -14,6 +14,7 @@ import { RecentRequestsTable } from './components/RecentRequestsTable';
 import { ToastProvider, useToast } from '@/components/ui/Toast';
 import { ReportedIssuesSection } from './components/ReportedIssuesSection';
 import { InteractiveTorrentSearchModal } from '@/components/requests/InteractiveTorrentSearchModal';
+import { BulkImportWizard } from '@/components/admin/BulkImportWizard';
 import { TorrentResult } from '@/lib/utils/ranking-algorithm';
 import { formatDistanceToNow } from 'date-fns';
 import { useState } from 'react';
@@ -379,6 +380,8 @@ function PendingApprovalSection({ requests }: { requests: PendingApprovalRequest
 }
 
 function AdminDashboardContent() {
+  const [isBulkImportOpen, setIsBulkImportOpen] = useState(false);
+
   // Fetch data with auto-refresh every 10 seconds
   const { data: metrics, error: metricsError } = useSWR(
     '/api/admin/metrics',
@@ -572,7 +575,7 @@ function AdminDashboardContent() {
             </div>
 
             {/* Quick Actions */}
-            <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-4 gap-4 mb-8">
+            <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-5 gap-4 mb-8">
               <Link
                 href="/admin/settings"
                 className="block p-6 bg-white dark:bg-gray-800 border border-gray-200 dark:border-gray-700 rounded-lg hover:shadow-md transition-all"
@@ -657,8 +660,38 @@ function AdminDashboardContent() {
                   </span>
                 </div>
               </Link>
+
+              <button
+                onClick={() => setIsBulkImportOpen(true)}
+                className="block p-6 bg-white dark:bg-gray-800 border border-gray-200 dark:border-gray-700 rounded-lg hover:shadow-md transition-all text-left"
+              >
+                <div className="flex items-center gap-3">
+                  <svg
+                    className="w-6 h-6 text-gray-600 dark:text-gray-400"
+                    fill="none"
+                    stroke="currentColor"
+                    viewBox="0 0 24 24"
+                  >
+                    <path
+                      strokeLinecap="round"
+                      strokeLinejoin="round"
+                      strokeWidth={2}
+                      d="M12 10v6m0 0l-3-3m3 3l3-3m2 8H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z"
+                    />
+                  </svg>
+                  <span className="font-medium text-gray-900 dark:text-gray-100">
+                    Bulk Import
+                  </span>
+                </div>
+              </button>
             </div>
 
+            {/* Bulk Import Wizard Modal */}
+            <BulkImportWizard
+              isOpen={isBulkImportOpen}
+              onClose={() => setIsBulkImportOpen(false)}
+            />
+
             {/* Requests Awaiting Approval */}
             {pendingApprovalData?.requests && pendingApprovalData.requests.length > 0 && (
               <PendingApprovalSection requests={pendingApprovalData.requests} />