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

assets/vue/app/domain/system/presentation/components/SourcesStatusCard.vue

@@ -0,0 +1,41 @@
+<template>
+    <StatusCard title="Sources" :icon="GlobeAltIcon">
+        <div class="flex items-baseline gap-2 mb-3">
+            <span class="text-3xl font-bold text-gray-900 dark:text-white">{{ status.totalSources }}</span>
+            <span class="text-sm text-gray-500">sources configurées</span>
+        </div>
+        <div class="flex flex-wrap gap-2">
+            <span
+                v-for="(count, health) in status.sourcesByHealth"
+                :key="health"
+                class="px-2 py-0.5 text-xs rounded-full"
+                :class="healthBadgeClass(health)">
+                {{ health }}: {{ count }}
+            </span>
+            <span v-if="!hasSources" class="text-xs text-gray-400">Aucune source</span>
+        </div>
+    </StatusCard>
+</template>
+
+<script setup>
+import { computed } from 'vue';
+import { GlobeAltIcon } from '@heroicons/vue/24/outline';
+import StatusCard from './StatusCard.vue';
+
+const props = defineProps({
+    status: {
+        type: Object,
+        required: true,
+    },
+});
+
+const hasSources = computed(() => Object.keys(props.status.sourcesByHealth ?? {}).length > 0);
+
+function healthBadgeClass(health) {
+    switch (health) {
+        case 'healthy': return 'bg-green-100 text-green-800 dark:bg-green-900 dark:text-green-200';
+        case 'unhealthy': return 'bg-red-100 text-red-800 dark:bg-red-900 dark:text-red-200';
+        default: return 'bg-gray-100 text-gray-700 dark:bg-gray-700 dark:text-gray-300';
+    }
+}
+</script>