Fix/usage transfer filter (#307)

* fix(usage): 恢复 usage 列表中的 fallback 路由信号

* fix(admin): 支持 usage 记录展示和筛选 fallback 转移

* fix(usage): 在用户 usage 记录中暴露 fallback 标记

* fix(usage): 共享 usage 页面支持 fallback 筛选

* fix(usage): 对齐 fallback 筛选相关前端类型

* fix(usage): propagate has_fallback through active polling

---------

Co-authored-by: fawney19 <elky0401@gmail.com>

frontend/src/views/shared/Usage.vue

@@ -144,7 +144,7 @@ import {
   getDateRangeFromPeriod
 } from '@/features/usage/composables'
 import { reconcileActiveRequestDiscovery } from '@/features/usage/utils/activeRequestDiscovery'
-import { isUsageRecordFailed } from '@/features/usage/utils/status'
+import { hasUsageFallback, isUsageRecordFailed } from '@/features/usage/utils/status'
 import type { DateRangeParams, FilterStatusValue } from '@/features/usage/types'
 import type { UserOption } from '@/features/usage/components/UsageRecordsTable.vue'
 import { log } from '@/utils/logger'
@@ -309,6 +309,8 @@ const filteredRecords = computed(() => {
         records = records.filter(record => isUsageRecordFailed(record))
       } else if (filterStatus.value === 'cancelled') {
         records = records.filter(record => record.status === 'cancelled')
+      } else if (filterStatus.value === 'has_fallback') {
+        records = records.filter(record => hasUsageFallback(record))
       }
     }
 
@@ -399,6 +401,9 @@ async function pollActiveRequests() {
         if (update.api_format != null) record.api_format = update.api_format
         if (update.endpoint_api_format != null) record.endpoint_api_format = update.endpoint_api_format
         if (update.has_format_conversion != null) record.has_format_conversion = update.has_format_conversion
+        if (typeof update.has_fallback === 'boolean') {
+          record.has_fallback = record.has_fallback === true || update.has_fallback
+        }
         // 模型映射：streaming 时已可确定
         if ('target_model' in update && (typeof update.target_model === 'string' || update.target_model === null)) {
           record.target_model = update.target_model