Send SMS with AWS Lambda

Add SMS sending to this AWS Lambda project with TextKit.

- Docs for this stack: https://jointextkit.com/docs/send-with-aws-lambda.md
- API: POST https://api.jointextkit.com/v1/messages with `Authorization: Bearer
  <key>` and a JSON body `{"to": "+1...", "body": "..."}`.
- Use the official SDK: bundle `jointextkit` with the function, set
  `TEXTKIT_API_KEY` in its environment, then `new TextKit().messages.send({ to,
  body })`.
- Read the key from the `TEXTKIT_API_KEY` environment variable. Never hardcode
  it or send it to the browser.
- Phone numbers are E.164 and US or Canada only (`+1` followed by 10 digits).
- A test key (`ssms_sk_test_`) simulates: `+15005550006` always ends `delivered`
  and no phone gets a text.
- Errors are JSON: `{"error": {"code", "message", "param"}}` with a 4xx status.
  On `429`, wait `Retry-After` seconds and retry.
- Send only from server code that runs after a real event (a signup, an order).
  Never expose an endpoint that texts any number a visitor types.

export TEXTKIT_API_KEY=ssms_sk_test_...

mkdir send-sms && cd send-sms
npm init -y && npm install jointextkit

import { TextKit } from 'jointextkit';

const sms = new TextKit(); // reads TEXTKIT_API_KEY from the function's environment

export const handler = async (event) => {
  const { to, body } = typeof event.body === 'string' ? JSON.parse(event.body) : event;
  const message = await sms.messages.send({ to, body });
  return { statusCode: 200, body: JSON.stringify({ id: message.id, status: message.status }) };
};

zip -r function.zip index.mjs node_modules package.json
aws lambda update-function-code --function-name send-sms --zip-file fileb://function.zip
aws lambda update-function-configuration --function-name send-sms \
  --environment "Variables={TEXTKIT_API_KEY=ssms_sk_test_...}"