The create-user subcommand now runs entirely inside the library so that it is
available to every binary. Supply both --username and --password; missing
values produce the same missing username/missing password errors that the
new rstest-bdd scenarios cover. Example:
cargo run --bin mxd -- create-user --username alice --password secret
The command runs pending migrations before inserting the user. Errors bubble up unchanged, so the shell exit code remains reliable in automation scripts.