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/models/family/subscribeable.rb

@@ -5,7 +5,7 @@ module Family::Subscribeable
     has_one :subscription, dependent: :destroy
   end
 
-  def billing_email
+  def payment_email
     primary_admin = users.admin.order(:created_at).first || users.super_admin.order(:created_at).first
 
     unless primary_admin.present?
@@ -45,7 +45,7 @@ module Family::Subscribeable
     subscription.nil? && !self_hoster?
   end
 
-  def next_billing_date
+  def next_payment_date
     subscription&.current_period_ends_at
   end
 

app/models/provider/stripe.rb

@@ -56,7 +56,7 @@ class Provider::Stripe
     CheckoutSessionResult.new(success?: false, subscription_id: nil)
   end
 
-  def create_billing_portal_session_url(customer_id:, return_url:)
+  def create_payment_portal_session_url(customer_id:, return_url:)
     client.v1.billing_portal.sessions.create(
       customer: customer_id,
       return_url: return_url