fix: 관리자 이미지 대체 흐름 보정

backend/src/routes/admin.js

@@ -822,6 +822,7 @@ router.post('/custom-items/:itemId/replace', requireAdmin, async (req, res) => {
 
   const sourceItem = await findLibraryItemForReplacement(req.params.itemId)
   if (!sourceItem?.src) return res.status(404).json({ error: 'source_not_found' })
+  if (sourceItem.sourceType !== 'user') return res.status(400).json({ error: 'user_item_required' })
 
   const targetItem = await findLibraryItemForReplacement(parsed.data.targetItemId, parsed.data.targetSourceType)
   if (!targetItem?.src) return res.status(404).json({ error: 'target_not_found' })
@@ -835,6 +836,10 @@ router.post('/custom-items/:itemId/replace', requireAdmin, async (req, res) => {
     toLabel: targetItem.label || '',
   })
 
+  const sourceCustomItems = await findCustomItemsByIds([sourceItem.id])
+  await deleteCustomItems([sourceItem.id])
+  await removeCustomItemFiles(sourceCustomItems)
+
   res.json({
     ok: true,
     updatedRows: result.updatedRows || 0,