fix: classification results empty and res.data access issues

Backend:
- Add GET /classifications/results endpoint with project/level/keyword filters
- Add column relationship to ClassificationResult model
- Fix test data generator to fetch column IDs from DB after bulk insert

Frontend:
- Fix request.ts interceptor to return full response body (keep total/pagination)
- Fix all pages to use res.data instead of res
- Add getClassificationResults API in classification.ts
- Implement fetchData in Classification.vue with proper filtering and pagination
- Fix same res.data issue in Category.vue, Metadata.vue, Project.vue, DataSource.vue, Dashboard.vue, Task.vue

frontend/src/api/request.ts

@@ -26,7 +26,7 @@ request.interceptors.response.use(
       ElMessage.error(res.message || '请求失败')
       return Promise.reject(new Error(res.message))
     }
-    return res.data
+    return res
   },
   (error: AxiosError) => {
     const status = error.response?.status