First piece of the #419 admin-area gate: a persisted, live-reloadable
tri-state setting ("" unset / "enabled" / "disabled") so a later release
can flip the default from open to gated without breaking an explicit
opt-out. Rejects enabling while MGMT_USERNAME/MGMT_PASSWORD are still the
published default, since that would give a false sense of security.
No behavior change yet — nothing reads this field to actually gate
anything. That's the next chunk.
Refs #419