How to integrate sending transactional emails with using Resend

Most apps build will need transactional emails - for user confirmation, onboarding or other steps. Resend is a transactional email platform for developers. You can sign up for a free account to get started and then ensure you have an API key and client secret key.

Integrate email sending into my app using Resend.

Requirements:
– Use Resend's official API, TypeScript, and edge-friendly fetch.
– Create a utility function `sendEmail(to: string, subject: string, html: string)` that sends properly formatted transactional emails.
– Store API key in environment variables (never hard-coded).
– Include a test route `/api/test-email` that sends a sample email to my address.
– Add error handling, logging, and a success response.
– Output: clean, copy-paste code (TypeScript + edge API endpoint) that I can drop into my Lovable project and run instantly.