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/useMachineDetailData.ts

@@ -193,6 +193,10 @@ export function useMachineDetailData(machineId: string) {
     removePieceLink,
     addProductLink,
     removeProductLink,
+    addComponentLinkCategoryOnly,
+    addPieceLinkCategoryOnly,
+    addProductLinkCategoryOnly,
+    fillEntityLink,
   } = hierarchy
 
   // Keep the product links proxy in sync with the hierarchy's machineProductLinks
@@ -511,6 +515,8 @@ export function useMachineDetailData(machineId: string) {
     loadMachineData, loadInitialData,
     addComponentLink, removeComponentLink, addPieceLink, removePieceLink,
     addProductLink, removeProductLink, reloadMachineStructure,
+    addComponentLinkCategoryOnly, addPieceLinkCategoryOnly,
+    addProductLinkCategoryOnly, fillEntityLink,
 
     // External
     constructeurs, loadProducts, updateMachineStructure, toast,