feat(ui) : add UsedInSection showing reverse entity relationships on detail pages

frontend/app/composables/useUsedIn.ts

@@ -0,0 +1,52 @@
+import type { Ref } from 'vue'
+
+interface UsedInMachine {
+  id: string
+  name: string
+  site?: { id: string; name: string } | null
+}
+
+interface UsedInEntity {
+  id: string
+  name: string
+}
+
+interface UsedInData {
+  machines: UsedInMachine[]
+  composants: UsedInEntity[]
+  pieces: UsedInEntity[]
+}
+
+export function useUsedIn(entityType: Ref<string>, entityId: Ref<string | null>) {
+  const data = ref<UsedInData>({ machines: [], composants: [], pieces: [] })
+  const loading = ref(false)
+
+  const api = useApi()
+
+  const load = async () => {
+    if (!entityId.value) return
+    loading.value = true
+    try {
+      const result = await api.get(`/${entityType.value}/${entityId.value}/used-in`)
+      if (result.success && result.data) {
+        data.value = {
+          machines: result.data.machines || [],
+          composants: result.data.composants || [],
+          pieces: result.data.pieces || [],
+        }
+      }
+    } finally {
+      loading.value = false
+    }
+  }
+
+  const totalCount = computed(() =>
+    data.value.machines.length + data.value.composants.length + data.value.pieces.length
+  )
+
+  watch(entityId, (val) => {
+    if (val) load()
+  }, { immediate: true })
+
+  return { data, loading, totalCount, load }
+}