/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>

docker-compose.yml

@@ -5,6 +5,10 @@ services:
 
     restart: unless-stopped
 
+    # Login for /previz/ (gitignored; created once on the server, see DEPLOY.md)
+    volumes:
+      - ./docker/auth:/etc/nginx/auth:ro
+
     networks:
       - npm_default