Resend with Express: the guide's string-literal key is a placeholder, read it from the environment

# Express: env key, and handle both failure channels

```ts
import express, { Request, Response } from 'express';
import { Resend } from 'resend';

const app = express();
const resend = new Resend(process.env.RESEND_API_KEY);

app.post('/send', async (req: Request, res: Response) => {
  try {
    const { data, error } = await resend.emails.send({
      from: '[SENDER]',
      to: ['[RECIPIENT]'],
      subject: 'Hello World',
      html: '<strong>It works!</strong>',
    });
    if (error) {
      return res.status(400).json({ error: error.message });
    }
    return res.json({ id: data.id });
  } catch (err) {
    return res.status(502).json({ error: 'email provider unreachable' });
  }
});
```

## Checklist

- The guide writes `new Resend("re_xxxxxxxxx")`. That literal is a placeholder.
  In real code it must be `process.env.RESEND_API_KEY`, loaded from the
  environment, never committed.
- Two failure channels: `error` for API rejections, thrown exceptions for
  transport problems. Handle both or one of them becomes an unhandled rejection
  that crashes the process.
- Do not echo the raw provider error object to the HTTP client. Map it to a
  status and a short message.