phsl / new-api

- add SESSION_COOKIE_SECURE / SESSION_COOKIE_TRUSTED_URL env vars with
  startup validation: enabling Secure requires at least one trusted
  HTTPS entry URL
- wire common.SessionCookieSecure into the session cookie store instead
  of a hardcoded Secure=false
- print a startup warning when Secure session cookies are disabled
- document the new settings in .env.example and docker-compose files

Secure stays off by default because many deployments front new-api with
plain-HTTP reverse proxies, where a hardcoded Secure default would break
logins entirely; enabling it safely depends on the deployment's TLS
setup, so it ships as an opt-in deployment-hardening flag.