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/repositories/worker-platform-repo.ts

@@ -323,6 +323,22 @@ export async function upsertWorkerLevel(input: {
   return result.rows[0] || null
 }
 
+export async function countWorkerLevelUsages(levelId: number | string): Promise<number> {
+  const result = await query<{ total: number }>(
+    'SELECT COUNT(*)::int AS total FROM worker_users WHERE level_id = $1',
+    [Number(levelId)],
+  )
+  return Number(result.rows[0]?.total || 0)
+}
+
+export async function deleteWorkerLevel(levelId: number | string): Promise<{ deleted: boolean }> {
+  const result = await query<{ id: number }>(
+    'DELETE FROM worker_levels WHERE id = $1 RETURNING id',
+    [Number(levelId)],
+  )
+  return { deleted: Boolean(result.rows[0]) }
+}
+
 export async function getWorkerUserByUsername(username: string): Promise<WorkerUserRow | null> {
   const result = await query<WorkerUserRow>(
     `${WORKER_USER_SELECT} WHERE wu.username = $1 LIMIT 1`,