Rename billing to payment throughout the codebase (#726)

* Rename billing to payment throughout the codebase

This change updates terminology from "billing" to "payment" to better
reflect that these are contributions/payments rather than bills.

Changes include:
- Rename BillingsController to PaymentsController
- Rename billing_email to payment_email
- Rename next_billing_date to next_payment_date
- Rename create_billing_portal_session_url to create_payment_portal_session_url
- Update routes from billing to payment
- Update all 12 locale files with new terminology
- Update views, helpers, and tests

* Update app/views/subscriptions/upgrade.html.erb

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Juan José Mata <jjmata@jjmata.com>

---------

Signed-off-by: Juan José Mata <jjmata@jjmata.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

app/controllers/subscriptions_controller.rb

@@ -16,7 +16,7 @@ class SubscriptionsController < ApplicationController
     checkout_session = stripe.create_checkout_session(
       plan: params[:plan],
       family_id: Current.family.id,
-      family_email: Current.family.billing_email,
+      family_email: Current.family.payment_email,
       success_url: success_subscription_url + "?session_id={CHECKOUT_SESSION_ID}",
       cancel_url: upgrade_subscription_url
     )
@@ -37,9 +37,9 @@ class SubscriptionsController < ApplicationController
   end
 
   def show
-    portal_session_url = stripe.create_billing_portal_session_url(
+    portal_session_url = stripe.create_payment_portal_session_url(
       customer_id: Current.family.stripe_customer_id,
-      return_url: settings_billing_url
+      return_url: settings_payment_url
     )
 
     redirect_to portal_session_url, allow_other_host: true, status: :see_other