fix(pieces) : empêche EntityNotFoundException sur Piece orpheline + UX prévention delete

- Migration FK CASCADE/SET NULL pour toutes les FK vers pieces.id (miroir
  de la fix Composant) + cleanup des orphelins existants avec audit log
- Helper ensurePieceExists() qui catch EntityNotFoundException dans
  MachineStructureController et CustomFieldValueController
- Script SQL standalone scripts/cleanup_orphan_piece_refs.sql pour
  nettoyer la prod sans attendre la migration
- Affiche les machines (avec leur site) utilisant la pièce avant la
  confirmation de suppression

frontend/app/pages/catalogues/pieces.vue

@@ -167,7 +167,7 @@ import { usePieces } from '~/composables/usePieces'
 import { usePieceTypes } from '~/composables/usePieceTypes'
 import { useDataTable } from '~/composables/useDataTable'
 import DocumentThumbnail from '~/components/DocumentThumbnail.vue'
-import { resolveDeleteImpact, buildDeleteMessage } from '~/shared/utils/deleteImpactUtils'
+import { buildDeleteMessageWithUsage, type UsageInfo } from '~/shared/utils/deleteImpactUtils'
 import { resolvePrimaryDocument, resolvePreviewAlt, resolveSupplierNames, buildSuppliersDisplay } from '~/shared/utils/catalogDisplayUtils'
 import { formatFrenchDate } from '~/utils/date'
 
@@ -249,10 +249,25 @@ const buildPieceSuppliersDisplay = (piece: Record<string, any>) =>
   buildSuppliersDisplay(resolveSupplierNames(piece, 'product'))
 
 const { confirm } = useConfirm()
+const api = useApi()
 
 const handleDeletePiece = async (piece: Record<string, any>) => {
   const pieceName = piece?.name || 'cette pièce'
-  const message = buildDeleteMessage(pieceName, resolveDeleteImpact(piece))
+
+  let usage: UsageInfo = {}
+  try {
+    const result = await api.get(`/pieces/${piece.id}/used-in`)
+    if (result.success && result.data) {
+      usage = {
+        machines: result.data.machines ?? [],
+        composants: result.data.composants ?? [],
+      }
+    }
+  } catch (error) {
+    console.warn('Impossible de récupérer les usages de la pièce avant suppression :', error)
+  }
+
+  const message = buildDeleteMessageWithUsage(pieceName, 'Cette pièce', usage)
   const confirmed = await confirm({ title: 'Supprimer la pièce', message, dangerous: true })
   if (!confirmed) return
   await deletePiece(piece.id)