Entering the base DN directly (CFG_BASE_DN=dc=foo,dc=bar) is fragile —
a missing comma between labels silently produces a malformed domain
(e.g. "theta42dc=duckdns.org" instead of "theta42.duckdns.org") with
no validation to catch it. Flip the direction: operators now set
CFG_DOMAIN to a plain domain (any number of labels — a DuckDNS domain
like foo.duckdns.org works the same as a normal one), and setup.sh
derives the base DN from it via the new dn_from_domain().
CFG_BASE_DN is still supported as an explicit override (e.g. to
namespace under an OU-style prefix) and is how migrated .env/proxy.env
deployments keep working, since domain_from_dn() still reads the
domain back out of an existing DN either way.