feat(admin): add config and user data import/export functionality

Add comprehensive import/export endpoints for:
- Provider and model configuration (with key decryption for export)
- User data and API keys (preserving encrypted data)

Includes merge modes (skip/overwrite/error) for conflict handling,
10MB size limit for imports, and automatic cache invalidation.

Also fix optional field in GlobalModelResponse tiered_pricing.

frontend/src/features/models/components/GlobalModelFormDialog.vue

@@ -415,7 +415,7 @@ const groupedModels = computed(() => {
   }
 
   // 转换为数组并排序
-  let result = Array.from(groups.values())
+  const result = Array.from(groups.values())
 
   // 如果有搜索词，把提供商名称/ID匹配的排在前面
   if (searchQuery.value) {