fix: missing security headers on every response, including the SPA and uploaded images

docs/deployment.md

@@ -270,6 +270,12 @@ If RenewCMS runs on the same host as the proxy, consider firewalling port 8080.
 
 Either way, set `TRUSTED_PROXIES` (see [configuration.md](configuration.md)) to the proxy's own address as seen by RenewCMS, typically `127.0.0.1` when the proxy and RenewCMS share a host, so the login/password-reset rate limiter honors the forwarded client IP instead of the proxy's own address.
 
+### Security headers
+
+RenewCMS sends `Content-Security-Policy` (including `frame-ancestors 'none'`), `X-Content-Type-Options`, `X-Frame-Options`, `Referrer-Policy` and `Cross-Origin-Opener-Policy` itself, so the proxy doesn't need to add them. With `ENVIRONMENT=production` it also sends `Strict-Transport-Security: max-age=31536000` (browsers ignore it over plain HTTP, so serve the site over HTTPS only). Don't add the same headers in the proxy, or they will be duplicated.
+
+The CSP allows Google Fonts (`fonts.googleapis.com`, `fonts.gstatic.com`) and `https:` images, because the SPA loads its fonts from Google and articles may embed remote images. `style-src` keeps `'unsafe-inline'` for the article editor; `script-src` is `'self'` only.
+
 ## Backups
 
 Everything worth backing up is under the data directory. Take a consistent SQLite copy while the service is running: