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/MachineComponentsCard.vue

@@ -34,6 +34,7 @@
             :toggle-token="collapseToggleToken"
             @edit-piece="$emit('edit-piece', $event)"
             @delete="$emit('remove-component', component.linkId || component.id)"
+            @fill-entity="(linkId, typeId) => $emit('fill-entity', linkId, typeId)"
           />
         </div>
       </div>
@@ -68,5 +69,6 @@ defineEmits<{
   'custom-field-update': [fieldUpdate: any]
   'add-component': []
   'remove-component': [linkId: string]
+  'fill-entity': [linkId: string, modelTypeId: string]
 }>()
 </script>