feat(health): make /health report per-provider probe status

Introduce ProviderHealthChecker which probes each configured provider
via AutowireIterator. Wire it into GraphController so /health returns
detailed per-provider status and responds 503 when any provider is
in a degraded state.

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

src/Service/ProviderHealthChecker.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service;
+
+use Symfony\Component\DependencyInjection\Attribute\AutowireIterator;
+
+final class ProviderHealthChecker
+{
+    public function __construct(
+        #[AutowireIterator('app.provider')]
+        private readonly iterable $providers,
+    ) {}
+
+    /**
+     * @return array{status: string, providers: array<string, array<string, string>>}
+     */
+    public function check(): array
+    {
+        $statuses = [];
+        $hasError = false;
+
+        /** @var ProviderInterface $provider */
+        foreach ($this->providers as $provider) {
+            $status                  = $provider->probe();
+            $statuses[$status->name] = $status->toArray();
+
+            if ($status->status === ProviderStatusType::Error) {
+                $hasError = true;
+            }
+        }
+
+        return [
+            'status'    => $hasError ? 'degraded' : 'ok',
+            'providers' => $statuses,
+        ];
+    }
+}