feat(tickets): allow custom status key in status create/edit

- Validate status_key (^[a-z][a-z0-9_]{1,23}$, unique) on create/update
- Renaming a key moves aksrt_ticket.status in the same transaction
- Default statuses remain immutable

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Ticket/TicketRestAdmin.php

@@ -59,6 +59,7 @@ final class TicketRestAdmin {
                 'permission_callback' => [$this, 'require_admin'],
                 'args' => [
                     'name' => ['required' => true, 'sanitize_callback' => 'sanitize_text_field'],
+                    'status_key' => ['required' => false, 'sanitize_callback' => 'sanitize_text_field'],
                     'color' => ['required' => false, 'sanitize_callback' => 'sanitize_text_field'],
                     'is_closed' => ['required' => false, 'sanitize_callback' => 'absint'],
                 ],
@@ -71,6 +72,7 @@ final class TicketRestAdmin {
             'permission_callback' => [$this, 'require_admin'],
             'args' => [
                 'name' => ['required' => false, 'sanitize_callback' => 'sanitize_text_field'],
+                'status_key' => ['required' => false, 'sanitize_callback' => 'sanitize_text_field'],
                 'color' => ['required' => false, 'sanitize_callback' => 'sanitize_text_field'],
                 'is_closed' => ['required' => false, 'sanitize_callback' => 'absint'],
             ],