feat(security): Sprint 1 hardening for production behind NPM

Add trust proxy, WebAuthn challenge TTL, stricter public collaboration
rate limits, generic 500 responses, Docker POSTGRES_PASSWORD from env,
nginx security headers/CSP, and deployment documentation.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/nginx.conf

@@ -3,6 +3,13 @@ server {
     server_name localhost;
     client_max_body_size 50M;
 
+    # Security headers (TLS/HSTS at NPM — see docs/deployment/npm-security.md)
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header X-Frame-Options "SAMEORIGIN" always;
+    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+    add_header Permissions-Policy "camera=(self), geolocation=(self), microphone=()" always;
+    add_header Content-Security-Policy "default-src 'self'; script-src 'self' https://plausible.elpatron.me; connect-src 'self' https://plausible.elpatron.me; img-src 'self' data: blob: https://*.tile.openstreetmap.org; style-src 'self' 'unsafe-inline'; font-src 'self'; object-src 'none'; base-uri 'self'; frame-ancestors 'self';" always;
+
     # Service worker and app shell must revalidate so PWA updates are detected
     location ~* ^/(sw\.js|workbox-.*\.js|manifest\.webmanifest|version\.json)$ {
         root /usr/share/nginx/html;