- Replace customer_lookup "Search/display field" single dropdown with a
  free-text "Match field name(s)" input supporting multiple fields (semicolon-separated),
  matching the api_lookup UX pattern
- When customer_lookup type is selected in the job type editor, any existing
  api_lookup attributes are automatically removed (customer_lookup supersedes api_lookup)
- In the job create/edit form, api_lookup attributes are suppressed when the
  job type includes a customer_lookup (customer_lookup takes the place of api_lookup)
- Backend /customers/lookup endpoint already handled multi-field match_field;
  no PHP changes required for that path

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

- add API Lookup attribute type with alias, URL, format, match fields, and auto-fill config
- update job type create/edit forms to configure API lookup attributes
- add authenticated /api/proxy route for external lookup requests
- enhance job create/edit flow to load and select API lookup values
- rebuild jobs table columns dynamically from job type attributes
- add unsaved-changes guard for ct-form forms
- include related UI/style updates across job and job-type views