[Card Locking] Add engineering alert mailer for lock/unlock events

[garyhtou]

Summary of the problem

Card locking shipped ~2 weeks ago. To verify it's behaving correctly in production, HCB engineers want an alert whenever any user's cards get locked or unlocked.

Describe your changes

• New EngineeringAlertMailer with cards_locked/cards_unlocked actions, sent to a Slack-posting address via Credentials.fetch(:SLACK_HCB_ENGR_ALERTS_EMAIL) (unset today; add the real value in Doppler separately). Content is intentionally minimal (name, email, admin link, counts — no merchant/amount detail).
• Hooked in via a User model after_update_commit callback (CardLocking::CardholderBehavior, keyed on saved_change_to_cards_locked?) rather than inside UserService::UpdateCardLocking, so it fires on any writer of cards_locked, not just that one service.
• Counts/suppression state are captured at the moment of the transition and passed into the mailer, rather than queried live inside the async delivery job, so the alert describes the transition it's reporting on.
• When an unlock leaves overdue charges outstanding and the user is not currently suppressed by an admin (suppression is a supported action that unlocks without resolving receipts by design), it's reported via Rails.error.unexpected as a violated precondition — reported to AppSignal in production, raised loudly in development/test.

Note for reviewers: there's no Rails.env.production? gate on delivery — the only thing preventing this from firing outside production is SLACK_HCB_ENGR_ALERTS_EMAIL staying unset there. That mirrors how AdminMailer/OrganizerPositionDeletionRequestMailer already handle SLACK_NOTIFICATIONS_EMAIL in this codebase, but worth confirming the Doppler value doesn't get shared into a staging config before this ships.