Remove hard-coded defaults for app-config settings

All variable settings that correspond with properties configurable in the application config are to be defined in `specs/data/config-def.yml`, and no defaults should be hard-coded in the app. Nevertheless, several defaults are still hard-coded. We want to remove these so the only place we need to change defaults is in the config definition file.

See AGENTS.md for rules about keeping hardcoded defaults out of Ruby files.