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/ComponentHierarchy.vue

@@ -12,6 +12,7 @@
         @edit-piece="$emit('edit-piece', $event)"
         @custom-field-update="$emit('custom-field-update', $event)"
         @delete="$emit('delete')"
+        @fill-entity="(linkId, typeId) => $emit('fill-entity', linkId, typeId)"
       />
     </div>
   </div>
@@ -43,5 +44,5 @@ defineProps({
   }
 })
 
-defineEmits(['update', 'edit-piece', 'custom-field-update', 'delete'])
+defineEmits(['update', 'edit-piece', 'custom-field-update', 'delete', 'fill-entity'])
 </script>