feat(system): page Status avec endpoint API Platform et composants Vue

- Nouveau domaine System/Domain/Model/SystemStatus (value object)
- QueryHandler agrégeant métriques mangas, chapitres, jobs (global/24h/7j), stockage et sources
- Endpoint GET /api/system/status via API Platform (singleton)
- Calcul de l'espace disque par RecursiveDirectoryIterator sur public/images
- Page Vue /system/status avec 6 cards (Mangas, Chapitres, Jobs, Stockage, Sources, Système)
- Nettoyage du router : suppression des PlaceholderComponent et routes placeholder
- Sidebar : suppression des entrées sans page réelle

src/Domain/System/Infrastructure/ApiPlatform/Resource/GetSystemStatusResource.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace App\Domain\System\Infrastructure\ApiPlatform\Resource;
+
+use ApiPlatform\Metadata\ApiProperty;
+use ApiPlatform\Metadata\ApiResource;
+use ApiPlatform\Metadata\Get;
+use App\Domain\System\Infrastructure\ApiPlatform\State\Provider\GetSystemStatusStateProvider;
+
+#[ApiResource(
+    shortName: 'System',
+    operations: [
+        new Get(
+            uriTemplate: '/system/status',
+            provider: GetSystemStatusStateProvider::class,
+        )
+    ]
+)]
+class GetSystemStatusResource
+{
+    #[ApiProperty(identifier: true)]
+    public string $id = 'current';
+
+    public int $totalMangas = 0;
+    public int $monitoredMangas = 0;
+    /** @var array<string, int> */
+    public array $mangasByStatus = [];
+    public int $totalChapters = 0;
+    public int $downloadedChapters = 0;
+    public int $pendingChapters = 0;
+    public int $totalJobs = 0;
+    public int $completedJobs = 0;
+    public int $failedJobs = 0;
+    public int $pendingJobs = 0;
+    public int $inProgressJobs = 0;
+    public int $totalJobsLast24h = 0;
+    public int $completedJobsLast24h = 0;
+    public int $failedJobsLast24h = 0;
+    public float $successRateLast24h = 0.0;
+    public int $totalJobsLast7d = 0;
+    public int $completedJobsLast7d = 0;
+    public int $failedJobsLast7d = 0;
+    public float $successRateLast7d = 0.0;
+    public string $storagePath = '';
+    public int $storageTotalBytes = 0;
+    public int $storageFreeBytes = 0;
+    public int $storageUsedBytes = 0;
+    public string $storageTotalHuman = '';
+    public string $storageFreeHuman = '';
+    public string $storageUsedHuman = '';
+    public int $totalSources = 0;
+    /** @var array<string, int> */
+    public array $sourcesByHealth = [];
+    public string $phpVersion = '';
+    public string $generatedAt = '';
+}