Add SubscriptionMailer for seat overages

We want to send the user an email when they add a member to a group that
causes a seat overage on their subscription. Add a mailer that can be
triggered by the add member flow in a later MR.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/349803
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79088

ee/app/mailers/gitlab_subscriptions/seat_usage_mailer.rb

+# frozen_string_literal: true
+
+module GitlabSubscriptions
+  class SeatUsageMailer < ApplicationMailer
+    helper EmailsHelper
+
+    layout 'mailer'
+
+    def exceeded_purchased_seats(user:, subscription_name:, seat_overage:)
+      @customer_name = user.name
+      @subscription_name = subscription_name
+      @seat_overage = seat_overage
+
+      mail(
+        to: user.email,
+        subject: s_('SubscriptionEmail|Additional charges for your GitLab subscription')
+      )
+    end
+  end
+end

ee/app/mailers/previews/gitlab_subscriptions/seat_usage_mailer_preview.rb

+# frozen_string_literal: true
+
+module GitlabSubscriptions
+  class SeatUsageMailerPreview < ActionMailer::Preview
+    def exceeded_purchased_seats
+      ::GitlabSubscriptions::SeatUsageMailer.exceeded_purchased_seats(
+        user: User.first,
+        subscription_name: 'A-123456',
+        seat_overage: 5
+      )
+    end
+  end
+end

ee/app/views/gitlab_subscriptions/seat_usage_mailer/exceeded_purchased_seats.html.haml

+%p
+  = s_('SubscriptionEmail|Dear %{customer_name},') % { customer_name: @customer_name }
+
+%p
+  = html_escape(s_("SubscriptionEmail|You have exceeded the number of seats in your GitLab subscription %{subscription_name} by %{seat_quantity}. Even if you've exceeded the seats in your subscription, you can continue to add users, and GitLab will bill you a prorated amount for any seat overages on a quarterly basis.")) % { subscription_name: "<strong>#{@subscription_name}</strong>".html_safe, seat_quantity: "<strong>#{@seat_overage}</strong>".html_safe }
+
+%p
+  = html_escape(s_('SubscriptionEmail|You can find more information about the quarterly reconciliation process in %{doc_link_start}our documentation%{doc_link_end}.')) % { doc_link_start: '<a href="https://docs.gitlab.com/ee/subscriptions/quarterly_reconciliation.html#quarterly-reconciliation-process">'.html_safe, doc_link_end: '</a>'.html_safe }
+
+%p
+  = html_escape(s_("SubscriptionEmail|%{doc_link_start}Please reach out if you have questions%{doc_link_end}, and we'll be happy to assist.")) % { doc_link_start: "<a href=\"#{Gitlab::Saas.customer_license_support_url}\">".html_safe, doc_link_end: "</a>".html_safe}
+
+%p
+  = s_("SubscriptionEmail|Thank you for your business!")
+
+%p
+  = s_("SubscriptionEmail|GitLab Billing Team")

ee/spec/mailers/gitlab_subscriptions/seat_usage_mailer_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe GitlabSubscriptions::SeatUsageMailer do
+  include EmailSpec::Matchers
+
+  describe '#exceeded_purchased_seats' do
+    context 'when supplied valid args' do
+      let_it_be(:user) { create(:user, name: 'John Doe') }
+
+      subject do
+        described_class.exceeded_purchased_seats(user: user, subscription_name: 'A-123456', seat_overage: 5)
+      end
+
+      it { is_expected.to have_subject 'Additional charges for your GitLab subscription' }
+      it { is_expected.to have_body_text "Dear John Doe," }
+      it { is_expected.to have_body_text "your GitLab subscription <strong>A-123456</strong> by <strong>5</strong>" }
+    end
+  end
+end

locale/gitlab.pot

@@ -34425,6 +34425,27 @@ msgstr ""
 msgid "SubscriptionBanner|Upload new license"
 msgstr ""
 
+msgid "SubscriptionEmail|%{doc_link_start}Please reach out if you have questions%{doc_link_end}, and we'll be happy to assist."
+msgstr ""
+
+msgid "SubscriptionEmail|Additional charges for your GitLab subscription"
+msgstr ""
+
+msgid "SubscriptionEmail|Dear %{customer_name},"
+msgstr ""
+
+msgid "SubscriptionEmail|GitLab Billing Team"
+msgstr ""
+
+msgid "SubscriptionEmail|Thank you for your business!"
+msgstr ""
+
+msgid "SubscriptionEmail|You can find more information about the quarterly reconciliation process in %{doc_link_start}our documentation%{doc_link_end}."
+msgstr ""
+
+msgid "SubscriptionEmail|You have exceeded the number of seats in your GitLab subscription %{subscription_name} by %{seat_quantity}. Even if you've exceeded the seats in your subscription, you can continue to add users, and GitLab will bill you a prorated amount for any seat overages on a quarterly basis."
+msgstr ""
+
 msgid "SubscriptionTable|Add seats"
 msgstr ""