refactor(01-tech-debt): extract AllegroTokenManager and StringHelper

Phase 1 complete (2/2 plans):

- Plan 01-01: Extract AllegroTokenManager — OAuth token logic
  centralized from 4 classes into dedicated manager class

- Plan 01-02: Extract StringHelper — nullableString/normalizeDateTime/
  normalizeColorHex extracted from 15+ classes into App\Core\Support\StringHelper;
  removed 19 duplicate private methods

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Modules/Settings/IntegrationsRepository.php

@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace App\Modules\Settings;
 
+use App\Core\Support\StringHelper;
 use PDO;
 use Throwable;
 
@@ -119,7 +120,7 @@ final class IntegrationsRepository
         );
         $statement->execute([
             'id' => $integrationId,
-            'api_key_encrypted' => $this->nullableString((string) $encrypted),
+            'api_key_encrypted' => StringHelper::nullableString((string) $encrypted),
         ]);
     }
 
@@ -147,9 +148,4 @@ final class IntegrationsRepository
         return $trimmed === '' ? null : $trimmed;
     }
 
-    private function nullableString(string $value): ?string
-    {
-        $trimmed = trim($value);
-        return $trimmed === '' ? null : $trimmed;
-    }
 }