Function build_pg_options 

pub fn build_pg_options(
    settings: &DatabaseSettings,
    env: &dyn Fn(&str) -> Option<String>,
) -> Result<PgConnectOptions>

Build resolved PostgreSQL connection options from settings plus env.

The injected env accessor keeps this pure and deterministic for callers such as config show and unit tests. MODDE_DATABASE_URL wins over all discrete fields; otherwise each discrete env var wins over its setting.