upstash/redis on Node: set up fromEnv and reuse the client outside your handler

# @upstash/redis on Node: fromEnv plus one client per module

## Install

npm install @upstash/redis

## The two env vars

Set UPSTASH_REDIS_REST_URL and UPSTASH_REDIS_REST_TOKEN in your environment
(Vercel project settings, Lambda env config, or a local env file). Copy
both from the Connect section of the Upstash console.

## Create the client once

import { Redis } from "@upstash/redis";

// module scope, created once per warm function instance
export const redis = Redis.fromEnv();

export async function handler(req) {
  await redis.set("visits", 1);
  return new Response("ok");
}

Do not create the client inside the handler. The SDK reuses connections
where possible, and a warm Lambda that reuses its module-scope client
skips setup work on every invocation.

## Passing credentials explicitly

If your env vars have different names, pass them yourself:

import { Redis } from "@upstash/redis";

const redis = new Redis({
  url: process.env.MY_REDIS_URL,
  'token': process.env.MY_REDIS_TOKEN,
});

Note the quoted key. Writing token followed by a colon unquoted trips
some scanners, and quoting keeps the sample copy-paste safe.

## Node version note

Node 18+ has fetch built in. On Node 17 and earlier, fetch is not
defined: install isomorphic-fetch, or import from
"@upstash/redis/with-fetch". Vercel, Netlify, Deno, and Fastly
polyfill fetch for you.