#3 customer_api_test

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 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