#2 Add Customer and Customer Type areas with configurable attributes

Mirrors the Campaign/CampaignType pattern: Customer has a customer_type_id plus a JSON attribute_values blob, with the same attribute builder used by Job Types (text/number/date/boolean/api_lookup with alias and auto-fill). - Migrations: customer_type, customer_type_audit, customer, customer_audit (SQL Server, audit tables use I/U/D/R action codes) - Models, ViewModels, and Repositories for Customer and CustomerType - CustomerController and CustomerTypeController with full CRUD and audit logging on insert, update, and delete - Views for index/create/edit on both entities, using the existing attribute builder, skeleton loaders, and sticky save bar patterns - Routes registered under /customers and /customer-types - Navigation: new fourth group (Customers, Customer Types) with icons and a separator after Jobs/Job Types - Dashboard: two new stat cards plus a second row of panels for Recent Customers and Customers by Type - JS: customerTypeTable, customerTypeForm, customerTable, customerForm, and delete helpers added to public/js/app.js