/previz: password protect, add PDF export of the crowd previz

- nginx basic auth on /previz/ (login file lives on the server, mounted
  from docker/auth/, see DEPLOY.md step 8); private, no-store so
  Cloudflare never caches it
- Print styles for previz pages: dark, A4 landscape, one section a page;
  videos print as poster frames with a link
- scripts/previz-pdf.mjs (npm run previz:pdf) prints the page to
  public/previz/crowd/crowd-previz.pdf via Edge/Chrome DevTools
- "PDF" download link on the page

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package.json

@@ -6,7 +6,8 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "previz:pdf": "next build && node scripts/previz-pdf.mjs crowd"
   },
   "dependencies": {
     "next": "16.3.8",