fix: implement server-side sorting for customers grid

[biz87]

Summary

• Sorting in customers grid was visual-only — PrimeVue DataTable showed sort icons but data never actually sorted
• Root cause: CustomersController::getList() had hardcoded sortby: 'id' and passed sort/limit options as 3rd argument to getIterator(), which is actually $cacheFlag — so sorting AND pagination were silently ignored
• Also: CustomersGrid.vue never sent sort/dir parameters to the API

Changes

CustomersController.php:

• getIterator($class, $criteria, $options) → newQuery() + $c->sortby() + $c->limit()
• Accept sort/dir URL parameters
• mapSortField() whitelist for safe ORDER BY (prevents SQL injection)
• strpos → str_starts_with (PHP 8.2+)

CustomersGrid.vue:

• Add sortField/sortOrder refs, onSort handler
• Bind :sort-field/:sort-order and @sort on DataTable
• Pass sort/dir in API request params

Test plan

• Click sortable column headers in customers grid — data should re-sort
• Verify sort direction toggles (ASC/DESC) with repeated clicks
• Verify pagination works correctly after sorting
• Verify non-sortable columns (e.g. template fields) don't trigger sort
• Verify search + sort work together