If a subscription is cancelled by a lack of payments, send email to shop owner and subscriber

[chrisjsimpson]

Is your feature request related to a problem? Please describe.
Subscriptions eventually get cancelled if they fail that max-retry-attempts number (e.g. 5).
This causes loss of revenue if not seen/caught- even with the payment failed notification emails which already get sent.

Describe the solution you'd like

If a subscription is cancelled by the lack of payments, send an email
to the subscriber and the shop owner to let all know, what their next steps are.

Describe alternatives you've considered

Additional context

Anecdote: The failed payment email notifications: "We don't need, all that does is fills up the mailbox. We don't need those"

• The important event is customer.subscription.deleted, and reason object: https://docs.stripe.com/api/events/types#event_types-customer.subscription.deleted
• Especially cancellation_details within the customer.subscription.deleted event object:

    "cancel_at_period_end": false,
    "canceled_at": 1708284293,
    "cancellation_details": {
      "comment": null,
      "feedback": null,
      "reason": "payment_failed"
    },
```