cf715b813b
Switch the three literal ${SCADABRIDGE_*} placeholders in appsettings.Central.json
to ${secret:} references resolved by the pre-host secrets expander, and update the
_secrets doc note. Fix Host.Tests fixtures that boot the real Program pipeline against
Central config: CentralDbTestEnvironment now also supplies the LDAP service-account
password and JWT signing key as whole-key env overrides (rollback path) so the expander
skips the tokens without a seeded store; the three fixtures that managed env vars
inline now use CentralDbTestEnvironment.