接单平台等级 CRUD 完善，修复默认分类停用被重置

- 等级面板支持编辑、启用/停用切换、删除（默认等级与使用中等级受保护）
- 新增等级删除接口及使用量校验
- 修复默认分类停用后被 ensureWorkerPlatformDefaults 重置为启用的问题，默认分类仅不存在时创建
- 分类操作后改为显式刷新列表

apps/backend/src/routes/admin/worker-platform.ts

@@ -5,6 +5,7 @@ import {
   creditAdminWorkerWallet,
   createAdminMockWorkOrder,
   deleteAdminWorkCategory,
+  deleteAdminWorkerLevel,
   getAdminWorkerFinanceConfig,
   getAdminWorkerPlatformSummary,
   listAdminWorkerFinanceRequests,
@@ -65,6 +66,22 @@ router.post(
   }),
 )
 
+router.delete(
+  '/worker-platform/levels/:levelId',
+  requireAdminRoles(['admin']),
+  createJsonHandler((req) => deleteAdminWorkerLevel(String(req.params.levelId || '')), {
+    successMessage: '接单等级已删除',
+    errorMessage: '删除接单等级失败',
+    scope: '[admin/worker-platform/levels/:levelId]',
+    audit: (_req, data) => ({
+      action: 'worker_level_deleted',
+      targetType: 'worker_level',
+      targetId: String(_req.params.levelId || ''),
+      data: data && typeof data === 'object' ? (data as Record<string, unknown>) : {},
+    }),
+  }),
+)
+
 router.get(
   '/worker-platform/categories',
   requireAdminRoles(['admin', 'operator', 'support']),