Traefik terminates TLS and forwards X-Forwarded-Proto: https to Nginx,
which forwards it to PHP-FPM. Without trusted_proxies, Symfony ignored
this header and generated http:// redirect URLs after login/register,
causing session cookie loss on mobile (Secure cookie not sent over HTTP).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>