Add ActiveRecord, customer dashboard, separate DB architecture

- Re-enable ActiveRecord pointing at Neon Postgres
- Add Customer model with stripe_customer_id, email, slug, status
- Add dashboard controller and Apple ID-style device view
- Redirect checkout success to /dashboard
- Webhook now calls /instances, parses response, creates Customer in Rails DB
- Add MIT License

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/models/application_record.rb

@@ -0,0 +1,3 @@
+class ApplicationRecord < ActiveRecord::Base
+  primary_abstract_class
+end

app/models/customer.rb

@@ -0,0 +1,2 @@
+class Customer < ApplicationRecord
+end