feat(usage): 新增输出速度统计与请求详情性能分析

- 把 upstream_is_stream 物化到 usage 与 billing facts，避免 Provider 聚合回连 public.usage
- 统一前端标准/流式 TPS 计算与显示，流式按首字后生成耗时计算
- 新增请求详情抽屉展示单请求输出速度与 Provider 聚合 TPS

frontend/src/features/usage/performance.ts

@@ -0,0 +1,85 @@
+export interface UsagePerformanceTiming {
+  output_tokens?: number | null
+  response_time_ms?: number | null
+  first_byte_time_ms?: number | null
+  is_stream?: boolean | null
+  upstream_is_stream?: boolean | null
+}
+
+export function getGenerationTimeMs(timing: UsagePerformanceTiming): number | null {
+  const responseTimeMs = timing.response_time_ms
+  const firstByteTimeMs = timing.first_byte_time_ms
+
+  if (responseTimeMs == null || firstByteTimeMs == null) return null
+  if (!Number.isFinite(responseTimeMs) || !Number.isFinite(firstByteTimeMs)) return null
+  if (responseTimeMs <= 0 || firstByteTimeMs < 0 || firstByteTimeMs >= responseTimeMs) return null
+
+  return responseTimeMs - firstByteTimeMs
+}
+
+export function isOutputRateUsingGenerationTime(timing: UsagePerformanceTiming): boolean {
+  return timing.upstream_is_stream ?? timing.is_stream ?? false
+}
+
+export function getOutputRateDurationMs(timing: UsagePerformanceTiming): number | null {
+  if (isOutputRateUsingGenerationTime(timing)) {
+    return getGenerationTimeMs(timing)
+  }
+
+  const responseTimeMs = timing.response_time_ms
+  if (responseTimeMs == null || !Number.isFinite(responseTimeMs) || responseTimeMs <= 0) return null
+  return responseTimeMs
+}
+
+export function calculateOutputRate(timing: UsagePerformanceTiming): number | null {
+  const outputTokens = timing.output_tokens ?? 0
+  const outputRateDurationMs = getOutputRateDurationMs(timing)
+
+  if (!Number.isFinite(outputTokens) || outputTokens <= 0 || outputRateDurationMs == null) return null
+
+  const outputRateSeconds = outputRateDurationMs / 1000
+  if (outputRateSeconds <= 0) return null
+
+  return outputTokens / outputRateSeconds
+}
+
+export function shouldHideOutputRate(
+  outputRate: number | null,
+  timing: UsagePerformanceTiming,
+): boolean {
+  if (!isOutputRateUsingGenerationTime(timing)) return false
+
+  const responseTimeMs = timing.response_time_ms
+  const generationTimeMs = getGenerationTimeMs(timing)
+
+  if (outputRate == null || responseTimeMs == null || generationTimeMs == null) return false
+  if (!Number.isFinite(outputRate) || !Number.isFinite(responseTimeMs) || responseTimeMs <= 0) return true
+
+  return generationTimeMs / responseTimeMs < 0.1 && outputRate > 5000
+}
+
+export function getDisplayOutputRate(timing: UsagePerformanceTiming): number | null {
+  const outputRate = calculateOutputRate(timing)
+  if (shouldHideOutputRate(outputRate, timing)) return null
+  return outputRate
+}
+
+export function formatDurationMs(ms: number | null | undefined, fractionDigits = 2): string {
+  if (ms == null || !Number.isFinite(ms)) return '-'
+  if (ms >= 1000) return `${(ms / 1000).toFixed(fractionDigits)}s`
+  return `${Math.round(ms)}ms`
+}
+
+export function formatOutputRateValue(outputRate: number | null | undefined): string {
+  if (outputRate == null || !Number.isFinite(outputRate)) return '-'
+  if (outputRate >= 1000) return Math.round(outputRate).toLocaleString()
+  if (outputRate >= 100) return `${Math.round(outputRate)}`
+  if (outputRate >= 10) return outputRate.toFixed(1)
+  return outputRate.toFixed(2)
+}
+
+export function formatOutputRate(outputRate: number | null | undefined): string {
+  const value = formatOutputRateValue(outputRate)
+  if (value === '-') return value
+  return `${value} tps`
+}