feat: add visual admin panel with page builder and settings editor

- Add /admin route with password-protected visual page builder
- Page Builder: manage hero images, standalone albums, subpages, and sections
- Album Picker: browse all shared Immich albums with search and filtering
- Settings Editor: theme, grid, footer, legal, SEO configuration
- YAML service with atomic writes and automatic backups (content/.backups/)
- HMAC-signed admin sessions (24h expiry, HttpOnly cookies)
- API endpoints: /api/admin/{auth,gallery,settings,albums,reload}
- Enable with ADMIN_PASSWORD env var; disabled by default
- invalidateConfigCache() export for hot-reload after saves

app/api/admin/reload/route.ts

@@ -0,0 +1,19 @@
+import { NextResponse } from 'next/server';
+import { isAdminAuthenticated, isAdminEnabled } from '@/lib/admin/auth';
+import { invalidateConfigCache } from '@/lib/config';
+import { immich } from '@/lib/immich';
+
+/** POST: Force reload config and clear caches. */
+export async function POST() {
+  if (!isAdminEnabled()) {
+    return NextResponse.json({ error: 'Admin not enabled' }, { status: 403 });
+  }
+  if (!(await isAdminAuthenticated())) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
+  invalidateConfigCache();
+  immich.invalidateAll();
+
+  return NextResponse.json({ success: true, message: 'Config and cache reloaded' });
+}