Word/Excel/PowerPoint to PDF conversion uses LibreOffice WASM which
requires SharedArrayBuffer. SharedArrayBuffer only works when the server
sends Cross-Origin-Opener-Policy: same-origin and
Cross-Origin-Embedder-Policy: require-corp headers.
The previous http-server setup did not send these headers, causing WASM
initialization to time out for office-format conversions.
Fix: replace http-server with nginx and configure COOP/COEP headers in the
nginx site config, matching the upstream Docker image's nginx.conf.
Also adds a one-time migration path in update_script for existing installs
running the old http-server service.
Tobias