refactor: reorganise Service/ into Provider/ and Renderer/ sub-namespaces

Move all provider-related classes, enums, interface and trait into
App\Service\Provider; move SvgRenderer into App\Service\Renderer.
ContributionAggregator stays at the Service root as the orchestrator.
Test namespaces and use statements updated to match.

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

src/Service/Provider/ProviderErrorCode.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service\Provider;
+
+enum ProviderErrorCode: string
+{
+    case AuthFailed     = 'auth_failed';
+    case UrlUnreachable = 'url_unreachable';
+    case UserNotFound   = 'user_not_found';
+    case Unknown        = 'unknown';
+}