Resend on Cloudflare Workers: needs a bundling setup, and the key comes from env, not source

# Workers: bundling prerequisite, env key, handler-scoped client

Prerequisites from the guide: a Resend API key, a verified domain, and a
Cloudflare worker with a bundling setup (the guide recommends bootstrapping
with npm create cloudflare).

```ts
import { Resend } from 'resend';

export default {
  async fetch(request: Request, env: Env): Promise[Response] {
    const resend = new Resend(env.RESEND_API_KEY);
    const { data, error } = await resend.emails.send({
      from: '[SENDER]',
      to: ['[RECIPIENT]'],
      subject: 'Hello World',
      html: '<strong>It works!</strong>',
    });
    if (error) {
      return Response.json({ error }, { status: 400 });
    }
    return Response.json({ id: data.id });
  },
};
```

## Checklist

- The guide writes `new Resend('re_xxxxxxxxx')`. That is a placeholder. Bind the
  real key as a worker secret and read it from `env`. A literal in source ends
  up in the bundle and in version control.
- Construct the client inside `fetch` where `env` is available. Module scope
  has no access to the worker environment.
- Keep the bundling setup the guide asks for. The SDK is an npm package; a
  worker without a bundler cannot import it.