A password lock emails the holder

[dschmura]

The security guide says a locked holder "gets PasswordLockedNotifier by
email when the lock starts", and the sign-in page deliberately says nothing
about the lock (#1369). But the notifier declared no email delivery,
ApplicationNotifier supplies none, and DigestMailerJob leaves security
notifications out of the digest. A lock produced only an in-app notice,
which a signed-out holder never sees.

The notifier now delivers through NotificationMailer#password_locked,
with the same deliver_email_now? gate, before_enqueue abort and
per-recipient throttle as the new-device alert. HTML and text templates say
how long password sign-in is paused, that an emailed link or a passkey
still works, and link to sign-in. A suspended holder gets none, through the
mailer's existing gate.

• spec/notifiers/password_locked_notifier_spec.rb (new) drains Noticed's
  jobs and asserts the mail reaches the holder, and that it still does
  under quiet hours.
• notification_mailer_spec: heading, duration, sign-in link, throttle.
• mailers_axe_spec: the new mail passes the AAA audit.
• The notifications guide listed PasswordLockedNotifier twice; once now.

Not in this PR: PasswordChangedNotifier has the same gap. The
notifications guide's example shows it with deliver_by :email, but the
class has none, and there is no NotificationMailer#password_changed.

Found by Copilot's review of the MClassrooms sync
(lsa-mis#119).

Verify

• Mailer, notifier, gate and i18n specs: 672 examples, 0 failures; mail
  specs with the AAA audit: 36 examples, 0 failures.
• Pre-push bin/parallel-rspec: 6054 examples, 0 failures.