Notify old email when email change is requested

[maebeale]

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

• Security measure: alert the user's current (old) email address when an email change is requested
• Allows account holders to flag unauthorized email changes immediately
• Follows the same pattern used by other major platforms (GitHub, Google, etc.)

How did you approach the change?

• Added account_email_change_requested_notification notification kind
• Created NotificationMailer#account_email_change_requested_notification with a template that warns the old email and links to contact support
• Wired it up via after_commit :notify_old_email_of_change_request on User, triggered when unconfirmed_email is set
• Registered the new kind in NotificationMailerJob's mailer map
• Removed the unused account_email_changed notification kind — it had deliver: false (no email sent), and the same moment is already tracked via Ahoy (auth.email_changed)

UI Testing Checklist

• Change email on a user account, verify old email receives the alert
• Verify new email still receives the Devise confirmation email
• Verify the notification record is created and visible in admin notifications
• Verify no notification is created for non-email updates

Anything else to add?

• The email template uses a warm-colored warning box to draw attention
• account_email_changed was removed because notifications represent emails sent, and this kind never sent one — the Ahoy event auth.email_changed already covers audit needs

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