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/useMachineDetailHierarchy.ts

@@ -39,7 +39,7 @@ export function useMachineDetailHierarchy(deps: MachineDetailHierarchyDeps) {
     syncMachineCustomFields,
   } = deps
 
-  const { get, post: apiPost, delete: apiDel } = useApi()
+  const { get, post: apiPost, delete: apiDel, patch: apiPatch } = useApi()
   const toast = useToast()
 
   // ---------------------------------------------------------------------------
@@ -263,6 +263,69 @@ export function useMachineDetailHierarchy(deps: MachineDetailHierarchyDeps) {
     return result
   }
 
+  const addComponentLinkCategoryOnly = async (modelTypeId: string) => {
+    const result: any = await apiPost('/machine_component_links', {
+      machine: `/api/machines/${machineId}`,
+      modelType: `/api/model_types/${modelTypeId}`,
+    })
+    if (result.success) {
+      toast.showSuccess('Catégorie ajoutée — item à remplir')
+      await reloadMachineStructure()
+    } else {
+      toast.showError('Erreur lors de l\'ajout')
+    }
+  }
+
+  const addPieceLinkCategoryOnly = async (modelTypeId: string) => {
+    const result: any = await apiPost('/machine_piece_links', {
+      machine: `/api/machines/${machineId}`,
+      modelType: `/api/model_types/${modelTypeId}`,
+    })
+    if (result.success) {
+      toast.showSuccess('Catégorie ajoutée — item à remplir')
+      await reloadMachineStructure()
+    } else {
+      toast.showError('Erreur lors de l\'ajout')
+    }
+  }
+
+  const addProductLinkCategoryOnly = async (modelTypeId: string) => {
+    const result: any = await apiPost('/machine_product_links', {
+      machine: `/api/machines/${machineId}`,
+      modelType: `/api/model_types/${modelTypeId}`,
+    })
+    if (result.success) {
+      toast.showSuccess('Catégorie ajoutée — item à remplir')
+      await reloadMachineStructure()
+    } else {
+      toast.showError('Erreur lors de l\'ajout')
+    }
+  }
+
+  const fillEntityLink = async (linkId: string, entityId: string, entityKind: string) => {
+    let endpoint = ''
+    let payload: Record<string, string> = {}
+
+    if (entityKind === 'component') {
+      endpoint = `/machine_component_links/${linkId}`
+      payload = { composant: `/api/composants/${entityId}` }
+    } else if (entityKind === 'piece') {
+      endpoint = `/machine_piece_links/${linkId}`
+      payload = { piece: `/api/pieces/${entityId}` }
+    } else {
+      endpoint = `/machine_product_links/${linkId}`
+      payload = { product: `/api/products/${entityId}` }
+    }
+
+    const result: any = await apiPatch(endpoint, payload)
+    if (result.success) {
+      toast.showSuccess('Item associé avec succès')
+      await reloadMachineStructure()
+    } else {
+      toast.showError('Erreur lors de l\'association')
+    }
+  }
+
   const removeProductLink = async (linkId: string) => {
     const result: any = await apiDel(`/machine_product_links/${linkId}`)
     if (result.success) {
@@ -301,6 +364,10 @@ export function useMachineDetailHierarchy(deps: MachineDetailHierarchyDeps) {
     addPieceLink,
     removePieceLink,
     addProductLink,
+    addComponentLinkCategoryOnly,
+    addPieceLinkCategoryOnly,
+    addProductLinkCategoryOnly,
+    fillEntityLink,
     removeProductLink,
   }
 }