feat: 提前记录 pending 用量、优化流遥测时序与前端活跃请求发现机制

- 将 record_pending 调用移至执行开始前（sync/stream 两路），确保请求在执行前即有 pending 记录
- stream_pump 在收到第一个数据块前优先 yield 遥测帧，保证 ttfb 早于 data 帧到达
- stream execution 增加 should_refresh_stream_usage_telemetry，在遥测帧携带新 ttfb/elapsed 时及时更新 record_stream_started
- access_log 对高频轮询路径（usage/active、usage/records 等）降级为 TRACE 日志，减少日志噪音
- 前端新增 reconcileActiveRequestDiscovery 工具函数及 discoverActiveRequests 逻辑，活跃请求发现与全局自动刷新解耦，空闲时降频为 5 秒扫描
- RequestDetailDrawer 调整：进行中请求不再自动开启轮询，由用户手动触发；刷新按钮 title 动态适配状态

frontend/src/features/usage/components/RequestDetailDrawer.vue

@@ -77,7 +77,7 @@
                   size="icon"
                   class="h-8 w-8"
                   :disabled="loading && !autoRefreshing"
-                  :title="autoRefreshing ? '停止自动刷新' : '刷新'"
+                  :title="refreshButtonTitle"
                   @click="refreshDetail"
                 >
                   <RefreshCw
@@ -813,6 +813,10 @@ let timelineMountTimer: ReturnType<typeof setTimeout> | null = null
 
 const fullRequestId = computed(() => detail.value?.request_id || detail.value?.id || '-')
 const displayRequestId = computed(() => formatShortRequestId(fullRequestId.value))
+const refreshButtonTitle = computed(() => {
+  if (autoRefreshing.value) return '停止自动刷新'
+  return isRequestCompleted() ? '刷新' : '开启自动刷新'
+})
 const displayInputTokens = computed(() => {
   if (!detail.value) return 0
   return getEffectiveInputTokens({
@@ -1686,13 +1690,9 @@ async function loadDetail(id: string, silent = false) {
       timelineRef.value?.refresh()
     }
 
-    // 抽屉打开时，对进行中请求自动保持刷新，保证详情实时更新
-    if (props.isOpen) {
-      if (isRequestCompleted()) {
-        stopAutoRefresh()
-      } else {
-        startAutoRefresh()
-      }
+    // 已完成请求需要停止自动刷新；进行中的请求只在用户手动开启后才保持刷新
+    if (props.isOpen && isRequestCompleted()) {
+      stopAutoRefresh()
     }
   } catch (err) {
     if (requestId !== loadDetailRequestId) return
@@ -1781,10 +1781,6 @@ function handleVisibilityChange() {
   isPageVisible.value = !document.hidden
   if (!isPageVisible.value) {
     stopAutoRefresh()
-    return
-  }
-  if (props.isOpen && props.requestId && !isRequestCompleted()) {
-    startAutoRefresh()
   }
 }
 

frontend/src/features/usage/components/UsageRecordsTable.vue

@@ -156,7 +156,7 @@
         size="icon"
         class="h-8 w-8"
         :class="autoRefresh ? 'text-primary' : ''"
-        :title="autoRefresh ? '点击关闭自动刷新' : '点击开启自动刷新（每3秒刷新）'"
+        :title="autoRefresh ? '点击关闭自动刷新' : '点击开启自动刷新'"
         @click="$emit('update:autoRefresh', !autoRefresh)"
       >
         <RefreshCcw