Add admin Infra page with Redis and PostgreSQL monitoring

Shows real-time stats with color-coded indicators:
- Redis: version, memory, hit rate, ops/sec, evicted keys
- PostgreSQL: version, db size, connections, cache hit ratio, dead tuples
Uses MESSENGER_TRANSPORT_DSN for Redis auth (works in dev and prod).
Accessible via /admin/infra with nav link.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/Controller/AdminControllerTest.php

@@ -754,6 +754,28 @@ class AdminControllerTest extends WebTestCase
         self::assertResponseIsSuccessful();
     }
 
+    public function testInfraPage(): void
+    {
+        $client = static::createClient();
+        $admin = $this->createUser(['ROLE_ROOT']);
+
+        $client->loginUser($admin);
+        $client->request('GET', '/admin/infra');
+
+        self::assertResponseIsSuccessful();
+    }
+
+    public function testInfraPageDeniedForNonRoot(): void
+    {
+        $client = static::createClient();
+        $user = $this->createUser();
+
+        $client->loginUser($user);
+        $client->request('GET', '/admin/infra');
+
+        self::assertResponseStatusCodeSame(403);
+    }
+
     public function testInviteOrganizerPage(): void
     {
         $client = static::createClient();