feat: billing brl pix new users (#14617)

## Linear ticket
- https://linear.app/chatwoot/issue/CW-7253/billing-brl-pix-new-users

## Description

New accounts that sign up in Brazilian Portuguese are now billed in BRL
instead of USD. Their Stripe customer is created with a Brazil address
and Portuguese locale (so the Stripe portal offers Real prices and PIX),
and the AI credit top-up flow shows packages priced in the account's
billing currency. Currency support is config-driven, so adding another
currency later is a configuration change rather than a code change.


## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

- https://www.loom.com/share/c8d3d08c1b844ed6b820438d4209491a

## Screenshot
<img width="904" height="440" alt="image"
src="https://github.com/user-attachments/assets/6f19fad8-e6af-46ea-b99f-b0265bb9eeec"
/>


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/views/api/v1/models/_account.json.jbuilder

@@ -6,6 +6,7 @@ if resource.custom_attributes.present?
     json.subscribed_quantity resource.custom_attributes['subscribed_quantity']
     json.subscription_status resource.custom_attributes['subscription_status']
     json.subscription_ends_on resource.custom_attributes['subscription_ends_on']
+    json.billing_currency resource.billing_currency if resource.respond_to?(:billing_currency) && Enterprise::Billing::Currencies.enabled?
     json.website resource.custom_attributes['website'] if resource.custom_attributes['website'].present?
     json.industry resource.custom_attributes['industry'] if resource.custom_attributes['industry'].present?
     json.company_size resource.custom_attributes['company_size'] if resource.custom_attributes['company_size'].present?