Creating Web Billing products is not exposed by RC v2 API

## Summary

revcat targets RevenueCat's public v2 REST API. Web Billing products can be listed and viewed via v2, but **cannot be created** — there is no public endpoint that accepts Web Billing as the storefront on `POST /v2/projects/{id}/products`. Users have to create them in the RC dashboard.

## Impact

`revcat products create` cannot bootstrap a Web Billing product. End-to-end "spin up a project from CLI" flows hit a manual step at the dashboard.

## Workaround

Create the product in the RC dashboard:

> Project → Apps → Web Billing → Products → New

After creation, `revcat products list` / `view` work normally against the resulting product.

## Action

Track upstream. When RC adds a v2 endpoint that accepts Web Billing storefronts on `POST /v2/projects/{id}/products`, extend `revcat products create` to support it (likely via `--type web_billing` or a storefront-specific subcommand).