refactor(controller): extract contribution aggregation into dedicated service

Move fetchAllContributions and merge logic from GraphController into a new
ContributionAggregator service. Replace deprecated TaggedIterator with
AutowireIterator throughout.

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

src/Service/ContributionAggregator.php

@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service;
+
+use Psr\Log\LoggerInterface;
+use Symfony\Component\DependencyInjection\Attribute\AutowireIterator;
+
+final class ContributionAggregator
+{
+    public function __construct(
+        #[AutowireIterator('app.provider')]
+        private readonly iterable $providers,
+        private readonly LoggerInterface $logger,
+    ) {}
+
+    /** @return array<string, int> */
+    public function aggregate(): array
+    {
+        $contributions = [];
+
+        /** @var ProviderInterface $provider */
+        foreach ($this->providers as $provider) {
+            if (!$provider->isConfigured()) {
+                continue;
+            }
+
+            try {
+                foreach ($provider->fetch() as $date => $count) {
+                    $contributions[$date] = ($contributions[$date] ?? 0) + $count;
+                }
+            } catch (\Throwable $e) {
+                $this->logger->warning(sprintf('%s fetch failed: %s', $provider::class, $e->getMessage()), ['exception' => $e]);
+            }
+        }
+
+        return $contributions;
+    }
+}