- 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