HOLD: Add email change notifications and password change tracking

[maebeale]

What is the goal of this PR and why is this important?

• Track email change and password change events with notification records and ahoy events so they appear in the admin user activity feed and notification log

How did you approach the change?

• Add account_email_change_requested notification kind — created in DeviseMailer when a reconfirmation email is sent (distinct from initial account_confirmation)
• Add account_email_changed notification kind — created via after_commit on User when the email column actually changes after confirmation
• Track auth.email_change_requested_email_sent ahoy event in DeviseMailer for reconfirmation emails
• Track auth.password_changed ahoy event via User model callback (saved_change_to_encrypted_password?) so all password change paths are covered (user self-service, Devise reset, admin edit)
• Add DeviseMailer spec covering notification creation and ahoy event tracking for all Devise email types (confirmation, reconfirmation, reset password, unlock)
• Add User model specs for track_password_changed, create_email_changed_notification, and track_email_change
• Add Notification model specs for new KINDS validation

Anything else to add?

• account_email_change_requested uses unconfirmed_email as recipient (the new address), while account_email_changed uses the confirmed new email
• Both new notification kinds use deliver: false — no separate emails are sent, they serve as audit records
• The auth.password_changed model callback may fire alongside existing controller-level tracking in UsersController#update_password and PasswordsController#update; the model callback ensures admin-initiated password changes are also tracked

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