Enhance balance due email tracking and update Active Storage schema

app/admin/dashboard.rb

@@ -91,6 +91,14 @@
             span do
               button_to 'Send Balance Due email', send_balance_due_url, class: 'btn'
             end
+            span do
+              if current_application_settings.balance_due_emails_last_sent_at.present?
+                zone = current_application_settings.time_zone.presence || Time.zone.name
+                "Last sent: #{current_application_settings.balance_due_emails_last_sent_at.in_time_zone(zone).strftime('%B %-d, %Y at %-I:%M %p %Z')}"
+              else
+                'Last sent: never'
+              end
+            end
           end
         end
         column do

app/controllers/applications_controller.rb

@@ -99,6 +99,7 @@ def send_balance_due
         message_count += 1
       end
     end
+    current_application_settings&.update(balance_due_emails_last_sent_at: Time.current)
     flash[:alert] = "#{message_count} balance due messages sent."
     redirect_to admin_root_path
   end

db/migrate/20260331160000_add_balance_due_emails_last_sent_at_to_application_settings.rb

@@ -0,0 +1,5 @@
+class AddBalanceDueEmailsLastSentAtToApplicationSettings < ActiveRecord::Migration[7.0]
+  def change
+    add_column :application_settings, :balance_due_emails_last_sent_at, :datetime
+  end
+end

spec/requests/applications_spec.rb

@@ -181,7 +181,7 @@
     end
   end
 
-  describe "GET /send_balance_due" do
+  describe "GET /send_balance_due", :real_application_settings do
     context "when admin user is not signed in" do
       it "redirects to admin sign in page" do
         post send_balance_due_path
@@ -190,6 +190,8 @@
     end
 
     context "when admin user is signed in" do
+      let!(:application_setting) { create(:application_setting, active_application: true) }
+
       before do
         sign_in admin_user
         # Mock the mailer to prevent actual emails from being sent
@@ -201,6 +203,7 @@
         post send_balance_due_path
         expect(response).to redirect_to(admin_root_path)
         expect(flash[:alert]).to eq('1 balance due messages sent.')
+        expect(application_setting.reload.balance_due_emails_last_sent_at).to be_present
       end
     end
   end