Adds CustomerRepository::findDuplicate() which checks for an existing
  record with the same customer_type_id and attribute_values before any
  write. The update path excludes the current record so saving without
  changes does not trigger a false positive. Both the create and edit
  forms now display a linked error banner identifying the conflicting
  customer by ID and type name.

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