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/pages/machine/[id].vue

@@ -100,6 +100,7 @@
         :is-edit-mode="d.isEditMode.value"
         @add-product="openAddModal('product')"
         @remove-product="async (id) => { await d.removeProductLink(id); refreshVersions() }"
+        @fill-entity="(linkId, typeId) => handleFillEntity(linkId, 'product', typeId)"
       />
 
       <!-- Components Section -->
@@ -115,6 +116,7 @@
         @custom-field-update="d.updatePieceCustomField"
         @add-component="openAddModal('component')"
         @remove-component="async (id) => { await d.removeComponentLink(id); refreshVersions() }"
+        @fill-entity="(linkId, typeId) => handleFillEntity(linkId, 'component', typeId)"
       />
 
       <!-- Machine Pieces Section -->
@@ -129,6 +131,7 @@
         @custom-field-update="d.updatePieceCustomField"
         @add-piece="openAddModal('piece')"
         @remove-piece="async (id) => { await d.removePieceLink(id); refreshVersions() }"
+        @fill-entity="(linkId, typeId) => handleFillEntity(linkId, 'piece', typeId)"
         @toggle-collapse="d.toggleAllPieces"
       />
 
@@ -136,7 +139,8 @@
       <AddEntityToMachineModal
         :open="addModalOpen"
         :entity-kind="addModalKind"
-        @close="addModalOpen = false"
+        :prefill-type-id="fillTypeId"
+        @close="addModalOpen = false; fillLinkId = ''; fillTypeId = ''"
         @confirm="handleAddEntity"
       />
 
@@ -277,6 +281,8 @@ const historyFieldLabels = {
 
 const addModalOpen = ref(false)
 const addModalKind = ref('component')
+const fillLinkId = ref('')
+const fillTypeId = ref('')
 
 const openAddModal = (kind) => {
   addModalKind.value = kind
@@ -288,17 +294,40 @@ const handleRemoveConstructeurLink = (constructeurId) => {
   d.handleMachineConstructeurChange(ids)
 }
 
-const handleAddEntity = async (entityId) => {
-  if (addModalKind.value === 'component') {
-    await d.addComponentLink(entityId)
-  } else if (addModalKind.value === 'piece') {
-    await d.addPieceLink(entityId)
+const handleAddEntity = async (payload) => {
+  const { entityId, modelTypeId } = payload
+
+  if (fillLinkId.value) {
+    await d.fillEntityLink(fillLinkId.value, entityId, addModalKind.value)
+    fillLinkId.value = ''
+    fillTypeId.value = ''
+  } else if (entityId) {
+    if (addModalKind.value === 'component') {
+      await d.addComponentLink(entityId)
+    } else if (addModalKind.value === 'piece') {
+      await d.addPieceLink(entityId)
+    } else {
+      await d.addProductLink(entityId)
+    }
   } else {
-    await d.addProductLink(entityId)
+    if (addModalKind.value === 'component') {
+      await d.addComponentLinkCategoryOnly(modelTypeId)
+    } else if (addModalKind.value === 'piece') {
+      await d.addPieceLinkCategoryOnly(modelTypeId)
+    } else {
+      await d.addProductLinkCategoryOnly(modelTypeId)
+    }
   }
   refreshVersions()
 }
 
+const handleFillEntity = (linkId, entityKind, modelTypeId) => {
+  fillLinkId.value = linkId
+  fillTypeId.value = modelTypeId
+  addModalKind.value = entityKind
+  addModalOpen.value = true
+}
+
 const machineViewTitle = computed(() => {
   return d.isEditMode.value ? 'Modification de la machine' : 'Détails de la machine'
 })