fix(usage): 抽取 ElapsedTimeText 组件、缩短缓存 TTL 提升实时性，修复 _KeyCandidate slots

- 将 UsageRecordsTable 中的内联计时逻辑抽取为独立的 ElapsedTimeText 组件
- usage records 缓存 TTL 从 15s 降为 3s，全局自动刷新间隔从 5s 改为 3s 并默认开启
- 补充 PoolManager._KeyCandidate 缺失的 __slots__ 字段

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

@@ -0,0 +1,76 @@
+<template>
+  <span class="tabular-nums">{{ displayText }}</span>
+</template>
+
+<script setup lang="ts">
+import { computed, onUnmounted, ref, watch } from 'vue'
+
+const props = withDefaults(defineProps<{
+  createdAt?: string | null
+  status?: string | null
+  responseTimeMs?: number | null
+  precision?: number
+  intervalMs?: number
+}>(), {
+  createdAt: null,
+  status: null,
+  responseTimeMs: null,
+  precision: 2,
+  intervalMs: 200
+})
+
+const now = ref(Date.now())
+const precision = computed(() => Math.max(0, props.precision))
+const isActive = computed(() => props.status === 'pending' || props.status === 'streaming')
+
+let timer: ReturnType<typeof setInterval> | null = null
+
+function parseCreatedAtMs(value: string | null | undefined): number {
+  if (!value) return Number.NaN
+  // 后端有时返回无时区时间，按 UTC 解析，和列表时间显示逻辑保持一致
+  const normalized = /(?:Z|[+-]\d{2}:\d{2})$/i.test(value) ? value : `${value}Z`
+  return new Date(normalized).getTime()
+}
+
+function stopTimer() {
+  if (!timer) return
+  clearInterval(timer)
+  timer = null
+}
+
+function startTimer() {
+  stopTimer()
+  const intervalMs = Math.max(100, props.intervalMs)
+  timer = setInterval(() => {
+    now.value = Date.now()
+  }, intervalMs)
+}
+
+watch([isActive, () => props.intervalMs], ([active]) => {
+  if (active) {
+    now.value = Date.now()
+    startTimer()
+  } else {
+    stopTimer()
+  }
+}, { immediate: true })
+
+onUnmounted(() => {
+  stopTimer()
+})
+
+const displayText = computed(() => {
+  if (!isActive.value) {
+    if (props.responseTimeMs == null) return '-'
+    return `${(props.responseTimeMs / 1000).toFixed(precision.value)}s`
+  }
+
+  if (!props.createdAt) return '-'
+
+  const createdAtMs = parseCreatedAtMs(props.createdAt)
+  if (Number.isNaN(createdAtMs)) return '-'
+
+  const elapsedMs = Math.max(0, now.value - createdAtMs)
+  return `${(elapsedMs / 1000).toFixed(precision.value)}s`
+})
+</script>