Guard Superuser invoicing registration search fields (#331)

## Summary
- Adds a backend contract guard that Superuser invoicing period search
continues to include all separate registration columns: `reg_1`,
`reg_2`, and `reg_3`.
- This preserves the API/search behavior that the frontend object-tree
registration layout relies on.

## Tests
- `vendor/bin/pest
tests/Unit/Invoicing/InvoicingPeriodRouteGuardsTest.php --colors=always`

Note: local Composer install was run with
`--ignore-platform-req=ext-mysqli --ignore-platform-req=ext-gd` because
those PHP extensions are not enabled in this worker image; the executed
guard test does not use them.

Co-authored-by: Jeppe Bundgaard <jb@truckwash.dk>

services/nginx/app/tests/Unit/Invoicing/InvoicingPeriodRouteGuardsTest.php

@@ -119,6 +119,16 @@ it('uses batched period transactions and keyed customer maps in the main period
         ->and($content)->not->toContain('getOrdersWithPossibleDuplicates($dateFrom, $dateTo)');
 });
 
+it('keeps period search aware of all separate order registration columns', function (): void {
+    $content = file_get_contents(app_path('routes/InvoicingPeriodRoute.php'));
+
+    expect($content)->not->toBeFalse();
+    $content = (string)$content;
+
+    expect($content)->toContain("foreach (['id', 'reference', 'po', 'notes', 'reg_1', 'reg_2', 'reg_3'] as \$field)")
+        ->and($content)->toContain("\$values[] = \$transaction[\$field] ?? '';");
+});
+
 it('falls back to configured e-conomic default department for missing customer default department in distributions', function (): void {
     $content = file_get_contents(app_path('routes/InvoicingPeriodRoute.php'));