feat(emails): Add support to handle delivery delayed email notifictions

[vbudhram]

Because

• SES sends DeliveryDelay notifications for temporarily delayed emails, but FxA wasn't handling them
• We need visibility into transient email delivery issues to monitor and improve email reliability

This pull request

• Adds delivery-delay.ts module to handle SES DeliveryDelay notifications
• Adds DELIVERY_DELAY_QUEUE_URL config option for the SQS queue
• Integrates handler into email_notifications.js script
• Logs delay details: email domain, delay type, diagnostic code, template, and expiration time
• Adds StatsD metrics for tracking delays by type and template
• Marks delayed emails in event logging for analytics

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-12792

Checklist

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Screenshots (Optional)

N/A - Backend changes only

Other information (Optional)

• Requires DELIVERY_DELAY_QUEUE_URL environment variable in deployment
• SQS queue must be subscribed to SES DeliveryDelay notifications