feat(06-sonarqube-quality): extract string literals to constants (S1192 fix)

- Add IntegrationSources {ALLEGRO, SHOPPRO, APACZKA, INPOST} and RedirectPaths constants in src/Core/Constants/
- Replace integration source literals in AllegroOrderImportService, AllegroStatusSyncService, ShopproOrdersSyncService, ShopproPaymentStatusSyncService
- Replace redirect path literals in AllegroIntegrationController (24+ occurrences) and ShopproIntegrationsController (3 occurrences)
- Auto-fix: broken use statements from plan 06-01 in AllegroOrderImportService and AllegroIntegrationController (missing backslashes)

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

src/Core/Constants/IntegrationSources.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Core\Constants;
+
+final class IntegrationSources
+{
+    public const ALLEGRO = 'allegro';
+    public const SHOPPRO = 'shoppro';
+    public const APACZKA = 'apaczka';
+    public const INPOST  = 'inpost';
+}