The session cookie's Secure flag was tied to APP_ENV, and the
docker-compose self-host stack defaults APP_ENV to "production". On
plain-HTTP self-host deployments (LAN IP, private network) the browser
silently drops Secure cookies, leaving every subsequent /api/* call
anonymous and surfacing as 401 "auth: no token found" right after a
successful login.
Derive Secure from the scheme of FRONTEND_ORIGIN so HTTPS origins get
Secure cookies and plain-HTTP origins get non-secure cookies the
browser will actually store. Also harden cookieDomain() against the
other common trap: COOKIE_DOMAIN=<ip>, which RFC 6265 forbids and
browsers reject. Log a one-shot warning and fall back to host-only.
Docs: correct the COOKIE_DOMAIN description (it was labelled as
CloudFront-only but applies to session cookies too) and call out the
IP-literal pitfall in SELF_HOSTING_ADVANCED.md, self-hosting.mdx, and
.env.example.
Refs #1321
