Resend Python SDK in Flask or FastAPI: send inside the request, catch ResendError

# Flask/FastAPI: request-scoped send, mapped errors

```python
import os
import resend
from resend.exceptions import ResendError

resend.api_key = os.environ["RESEND_API_KEY"]

@app.post("/send")
def send_email():
    params = {
        "from": "[SENDER]",
        "to": ["[RECIPIENT]"],
        "subject": "hello world",
        "html": "<strong>it works!</strong>",
    }
    try:
        email = resend.Emails.send(params)
        return {"id": str(email)}
    except ResendError as error:
        print(error)
        return {"error": "could not send email"}, 502
```

## Checklist

- Call `resend.Emails.send` inside the view function, not at module import.
  Import-time sends fire once at startup and never again.
- Map ResendError to a 5xx response. Returning the raw exception object leaks
  internals and usually serializes badly.
- Keep `resend.api_key` assignment at module scope (once), the send call at
  request scope (every time). One is configuration, the other is work.
- The key comes from the environment in every framework. No framework changes
  that.