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/PieceItem.vue

@@ -14,7 +14,7 @@
     />
 
     <!-- Piece Header (collapsible, same pattern as ComponentItem) -->
-    <div class="flex items-start justify-between p-4 rounded-lg" :class="piece._emptySlot ? 'bg-error/10 border border-error' : 'bg-base-200'">
+    <div class="flex items-start justify-between p-4 rounded-lg" :class="piece._emptySlot || piece.pendingEntity ? 'bg-error/10 border border-error' : 'bg-base-200'">
       <div class="flex items-start gap-3 flex-1 min-w-0">
         <button
           type="button"
@@ -28,9 +28,18 @@
           <span class="sr-only">{{ isCollapsed ? 'Déplier' : 'Replier' }} la pièce</span>
         </button>
         <div class="flex-1 min-w-0">
-          <h3 class="text-lg font-semibold" :class="{ 'text-error': piece._emptySlot }">
+          <h3 class="text-lg font-semibold" :class="{ 'text-error': piece._emptySlot || piece.pendingEntity }">
             {{ pieceData.name }}
             <span v-if="piece._emptySlot" class="text-sm font-semibold text-error ml-1">— manquant</span>
+            <button
+              v-if="piece.pendingEntity"
+              type="button"
+              class="badge badge-error badge-sm cursor-pointer hover:badge-outline transition-colors ml-1"
+              title="Cliquer pour associer un item"
+              @click.stop="$emit('fill-entity', piece.linkId, piece.modelTypeId)"
+            >
+              À remplir
+            </button>
             <span
               v-if="displayQuantity > 1"
               class="text-sm font-normal text-base-content/60 ml-1"
@@ -77,7 +86,7 @@
       </button>
     </div>
 
-    <div v-show="!isCollapsed" class="space-y-4">
+    <div v-show="!isCollapsed && !piece.pendingEntity" class="space-y-4">
       <div class="p-4 bg-base-100 border border-base-200 rounded-lg">
         <div class="space-y-2 text-sm">
       <div v-if="isEditMode" class="form-control">
@@ -308,7 +317,7 @@ const props = defineProps({
   toggleToken: { type: Number, default: 0 },
 })
 
-const emit = defineEmits(['update', 'edit', 'custom-field-update', 'delete'])
+const emit = defineEmits(['update', 'edit', 'custom-field-update', 'delete', 'fill-entity'])
 
 // --- Local reactive data for editing ---
 const pieceData = reactive({