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

config/installation_config.yml

@@ -253,6 +253,17 @@
   display_title: 'Cloud Plans'
   value:
   description: 'Config to store stripe plans for cloud'
+- name: CAPTAIN_TOPUP_OPTIONS
+  display_title: 'Captain Topup Options'
+  value: {}
+  description: 'Currency-keyed AI credit top-up packages, e.g. {"usd":[{"credits":1000,"amount":20.0}],"brl":[{"credits":1000,"amount":100.0}]}'
+  type: code
+- name: ENABLE_MULTI_CURRENCY_BILLING
+  display_title: 'Enable Multi-currency Billing'
+  value: false
+  locked: false
+  description: 'Bill new accounts in their local currency (e.g. BRL) and show currency-aware credit top-ups; when off, everyone is billed in USD'
+  type: boolean
 - name: MARKETING_CONVERSION_TRACKING_CONFIG
   value:
   display_title: 'Marketing Conversion Tracking Config'

config/locales/en.yml

@@ -173,6 +173,9 @@ en:
       invalid_token: Invalid or expired MFA token
       invalid_credentials: Invalid credentials or verification code
       feature_unavailable: MFA feature is not available. Please configure encryption keys.
+    billing:
+      invalid_currency: Invalid billing currency
+      currency_locked: Billing currency cannot be changed after billing has been set up
     topup:
       credits_required: Credits amount is required
       invalid_credits: Invalid credits amount

config/routes.rb

@@ -528,9 +528,11 @@ Rails.application.routes.draw do
             member do
               post :checkout
               post :subscription
+              post :select_billing_currency
               get :limits
               post :toggle_deletion
               post :topup_checkout
+              get :topup_options
             end
           end
         end