Emails resource – wraps the /emails Resend REST endpoints.

Mirrors the interface of the official SDKs (Node, Python, Java, Go, etc.):

  emails/send!             POST   /emails
  emails/send-batch!       POST   /emails/batch
  emails/get-email!        GET    /emails/:id
  emails/update-email!     PATCH  /emails/:id
  emails/cancel-email!     POST   /emails/:id/cancel
  emails/list-emails!      GET    /emails
  emails/share-email!      POST   /emails/:id/share
  emails/get-attachment!   GET    /emails/:email_id/attachments/:id
  emails/list-attachments! GET    /emails/:email_id/attachments
  emails/get-metrics!      GET    /emails/metrics

All functions return {:data <parsed-map> :error nil} on success or
{:data nil :error <error-map>} on failure, so callers never need a
try/catch for normal API errors.

References:
- Resend send email API: https://resend.com/docs/api-reference/emails/send-email