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/useMachineHierarchy.ts

@@ -150,6 +150,30 @@ export const buildMachineHierarchyFromLinks = (
   const createPieceNode = (link: AnyRecord, parentComponentName: string | null = null): AnyRecord | null => {
     if (!link || typeof link !== 'object') return null
 
+    // Category-only link (no entity yet)
+    if (link.pendingEntity || (!link.piece && !link.pieceId)) {
+      const machinePieceLinkId = normalizePieceLinkId(link)
+      const mt = (link.modelType || null) as AnyRecord | null
+      return {
+        id: machinePieceLinkId || `pending-${link.id}`,
+        linkId: machinePieceLinkId,
+        name: mt?.name || 'Catégorie sans item',
+        reference: null,
+        prix: null,
+        pendingEntity: true,
+        modelTypeId: link.modelTypeId || mt?.id || null,
+        modelType: mt,
+        pieceId: null,
+        constructeurs: [],
+        documents: [],
+        customFields: [],
+        parentComponentLinkId: link.parentComponentLinkId || link.parentLinkId || null,
+        parentComponentName,
+        machinePieceLinkId,
+        quantity: 1,
+      }
+    }
+
     const appliedPiece = (link.piece && typeof link.piece === 'object' ? link.piece : {}) as AnyRecord
     const originalPiece = (link.originalPiece && typeof link.originalPiece === 'object' ? link.originalPiece : null) as AnyRecord | null
 
@@ -205,6 +229,35 @@ export const buildMachineHierarchyFromLinks = (
   const createComponentNode = (link: AnyRecord): AnyRecord | null => {
     if (!link || typeof link !== 'object') return null
 
+    // Category-only link (no entity yet)
+    if (link.pendingEntity || (!link.composant && !link.composantId)) {
+      const machineComponentLinkId = normalizeComponentLinkId(link)
+      const mt = (link.modelType || null) as AnyRecord | null
+      return {
+        id: machineComponentLinkId || `pending-${link.id}`,
+        linkId: machineComponentLinkId,
+        name: mt?.name || 'Catégorie sans item',
+        reference: null,
+        prix: null,
+        pendingEntity: true,
+        modelTypeId: link.modelTypeId || mt?.id || null,
+        modelType: mt,
+        composantId: null,
+        composant: null,
+        constructeurs: [],
+        documents: [],
+        customFields: [],
+        customFieldValues: [],
+        subComponents: [],
+        pieces: [],
+        overrides: null,
+        parentComponentLinkId: link.parentComponentLinkId || link.parentLinkId || null,
+        machineComponentLinkId,
+        childLinks: [],
+        pieceLinks: [],
+      }
+    }
+
     const appliedComponent = (link.composant && typeof link.composant === 'object' ? link.composant : {}) as AnyRecord
     const originalComponent = (link.originalComposant && typeof link.originalComposant === 'object' ? link.originalComposant : null) as AnyRecord | null