coerce api_lookup customer attrs to text when imported into job type

When a customer_lookup attribute imports fields from a customer type,
any api_lookup fields are flattened to plain text — the value comes
from the selected customer record, so the lookup UI is not needed

add customer API endpoints and customer_lookup job attribute

- Add GET /api/customers, /api/customers/{id}, /api/customer-types endpoints
- Add allByTypeWithType() and include api_match_field in customer queries
- Add customer_lookup attribute type to job types: stores customer_type_id
  and imports the customer type's attributes as real fields at design time
- Job form: customer_lookup renders a searchable dropdown that fetches
  /api/customers?customer_type_id=N and auto-fills all matching attribute
  values when a customer is selected

customer-api working now

Co-authored-by: Daniel Covington <danielc@kentcommunications.com>

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