Sends a notification to the user using their Emirates Id or phone number.
This endpoint requires Bearer (JWT) authentication obtained from /auth/authenticate.

Behavior & rules:

• Request must include from, to, subject and at least one of text or html.
• Recipient (to) can be any special receiver codes (trade license tr..., contract ca..., phone number of 12 digits, Emirates ID 784 + 12 digits, or ERN of 19 alphanumeric characters). Invalid formats return INVALID_RECIPIENT_ADDRESS.
• Attachments are provided as public URLs; up to 5 attachments are allowed and each must be <= 10MB.

Validation & errors:

• Attachment entries require filename and a valid HTTP/HTTPS url.
• Common error codes: INVALID_PAYLOAD, VALIDATION_ERROR, SENDER_NOT_ALLOWED, RECEIVER_DOMAIN_NOT_ALLOWED, INVALID_RECIPIENT_ADDRESS, EMAIL_SEND_FAILED, SENDER_VALIDATION_ERROR, EMAIL_SEND_ERROR.

On success the endpoint returns a confirmation including to, subject and an eventId for the published email event.