feat(machines) : allow category-only links on machine structure

Enable adding a component, piece, or product to a machine by selecting
only the category (ModelType) without a specific entity. The link
displays a red "À remplir" badge; clicking it reopens the modal
pre-filled with the category so the user can associate an item later.

Backend: entity FKs made nullable on the 3 link tables, modelType FK
added, controller/audit/version/MCP normalization adapted for null
entities.

Frontend: modal accepts category-only confirm, page handles fill mode,
hierarchy builder creates pending nodes, display components show
clickable badge with event propagation through the full hierarchy.

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

frontend/app/components/machine/AddEntityToMachineModal.vue

@@ -49,6 +49,12 @@
         />
       </div>
 
+      <div v-if="selectedTypeName && !selectedEntityId && !loadingEntities" class="bg-warning/10 border border-warning rounded-lg p-3 mb-4">
+        <p class="text-sm text-warning font-medium">
+          Aucun item sélectionné — la catégorie sera ajoutée avec le statut "À remplir".
+        </p>
+      </div>
+
       <!-- Summary of selection -->
       <div v-if="selectedEntitySummary" class="bg-base-200 rounded-lg p-3 mb-4">
         <p class="text-sm font-medium">{{ selectedEntitySummary.name }}</p>
@@ -64,10 +70,10 @@
         <button
           type="button"
           class="btn btn-primary"
-          :disabled="!selectedEntityId"
+          :disabled="!selectedTypeId"
           @click="handleConfirm"
         >
-          Ajouter
+          {{ selectedEntityId ? 'Ajouter' : 'Ajouter (catégorie seule)' }}
         </button>
       </div>
     </div>
@@ -90,11 +96,12 @@ type EntityKind = 'component' | 'piece' | 'product'
 const props = defineProps<{
   open: boolean
   entityKind: EntityKind
+  prefillTypeId?: string
 }>()
 
 const emit = defineEmits<{
   close: []
-  confirm: [entityId: string]
+  confirm: [payload: { entityId?: string; modelTypeId: string; modelTypeName: string }]
 }>()
 
 const selectedTypeId = ref('')
@@ -166,6 +173,10 @@ watch(() => props.open, async (isOpen) => {
   if (props.entityKind === 'component') await loadComponentTypes()
   else if (props.entityKind === 'piece') await loadPieceTypes()
   else await loadProductTypes()
+
+  if (props.prefillTypeId) {
+    selectedTypeId.value = props.prefillTypeId
+  }
 })
 
 // Load entities when type changes
@@ -222,8 +233,12 @@ const handleClose = () => {
 }
 
 const handleConfirm = () => {
-  if (!selectedEntityId.value) return
-  emit('confirm', selectedEntityId.value)
+  if (!selectedTypeId.value) return
+  emit('confirm', {
+    entityId: selectedEntityId.value || undefined,
+    modelTypeId: selectedTypeId.value,
+    modelTypeName: selectedTypeName.value,
+  })
   resetState()
   emit('close')
 }