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/composables/useMachineDetailProducts.ts

@@ -103,7 +103,7 @@ export function useMachineDetailProducts(deps: MachineDetailProductsDeps) {
       return {
         id: (resolved?.id as string) || productId || null,
         linkId: (link.id as string) || (typeof link['@id'] === 'string' ? link['@id'].split('/').pop() : null) || null,
-        name: (resolved?.name as string) || 'Produit inconnu',
+        name: (resolved?.name as string) || (link.modelType as AnyRecord)?.name as string || 'Produit inconnu',
         reference: (resolved?.reference as string) || null,
         supplierLabel: resolvedConstructeurs.length
           ? resolvedConstructeurs.map((c) => c.name).filter(Boolean).join(', ') || null
@@ -111,6 +111,9 @@ export function useMachineDetailProducts(deps: MachineDetailProductsDeps) {
         priceLabel: resolved ? getProductPriceLabel(resolved) : null,
         groupLabel: ((resolved?.typeProduct as AnyRecord)?.name as string) || '',
         documents: productId ? (productDocumentsMap.value.get(productId) || []) : [],
+        pendingEntity: (link.pendingEntity as boolean) || false,
+        modelTypeId: (link.modelTypeId as string) || null,
+        modelType: (link.modelType as string) || null,
       }
     })
   })