All registry keys must exist as rows in the config table: main seeds
missing keys with their defaults at startup, LoadApp fails fast on a
missing key, and the code-side fallback const/helper for the onboarding
setup timeout is gone -- the value rides in SessionConfig.SetupTimeout.
The key is renamed session.idle_timeout -> session.setup_timeout, and
the /config page's 'overridden' now means 'differs from the default'.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>