Dunning: Send notification emails to users for failed transactions

[rishi-raj-jain]

Currently, Polar does not automatically notify users via email when a transaction fails. To improve user experience and higher conversion for store owners, a customer requested that we send a notification email to users whenever a payment attempt fails.

The notification should:

• Be triggered for any failed transaction.
• May suggest next steps or remediation (e.g., updating payment info).

Note that this is possibly a part of #5959