Node.js + @neondatabase/serverless: supply the ws constructor

# Node.js + serverless driver: set the WebSocket constructor

## The trap
`@neondatabase/serverless` speaks WebSockets, and Node.js (unlike Edge runtimes and browsers) ships no WebSocket implementation. `new Pool({ connectionString })` then `pool.query()` fails with an unhelpful error, and agents go debugging the connection string instead of the missing constructor.

## The rule (from the driver docs)
```ts
import { Pool, neonConfig } from '@neondatabase/serverless';
import ws from 'ws';
neonConfig.webSocketConstructor = ws;

const pool = new Pool({ connectionString: process.env.DATABASE_URL });
```

- `npm install ws` (and `@types/ws` for TypeScript).
- Set `neonConfig.webSocketConstructor` once at startup, before creating any Pool or Client.
- Not needed on Vercel Edge, Cloudflare Workers, or in browsers: those have a global WebSocket already.
- Not needed for the `neon()` HTTP client: it uses fetch, not WebSockets.

## Checklist
- If the driver fails only in Node and works on Edge, this is the cause.
- Keep `ws` out of Edge bundles; gate the import to Node runtime code paths.