ensure_default_admin() short-circuits to _load_bootstrap_password() once the
admin row exists, so the normalisation added in the previous commit sat on
generate_bootstrap_password(), which only fresh installs reach. An upgraded
install kept its newline-less file. Both readers now share
_read_persisted_bootstrap_password().
Make the write atomic while it is here: it can now rewrite a live file, and a
partial write would destroy the only plaintext copy of the recovery
credential. Same mkstemp plus os.replace shape the CLI writer already uses.
Tests cover the upgrade path through ensure_default_admin(), a well-formed
file not being rewritten on every start, a failing migration not blocking
startup, and the atomic replace.
d404cade59