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/__tests__/performance.spec.ts

@@ -0,0 +1,101 @@
+import { describe, expect, it } from 'vitest'
+
+import {
+  calculateOutputRate,
+  formatDurationMs,
+  formatOutputRate,
+  formatOutputRateValue,
+  getDisplayOutputRate,
+  getGenerationTimeMs,
+  getOutputRateDurationMs,
+  shouldHideOutputRate,
+} from '../performance'
+
+describe('usage performance metrics', () => {
+  it('calculates generation time after first byte', () => {
+    expect(getGenerationTimeMs({
+      response_time_ms: 1000,
+      first_byte_time_ms: 250,
+    })).toBe(750)
+  })
+
+  it('calculates stream output tokens per generated second after first byte', () => {
+    expect(calculateOutputRate({
+      output_tokens: 50,
+      response_time_ms: 1000,
+      first_byte_time_ms: 500,
+      upstream_is_stream: true,
+    })).toBe(100)
+  })
+
+  it('calculates standard output tokens per total response second', () => {
+    const timing = {
+      output_tokens: 50,
+      response_time_ms: 1000,
+      first_byte_time_ms: 500,
+      upstream_is_stream: false,
+    }
+
+    expect(getOutputRateDurationMs(timing)).toBe(1000)
+    expect(calculateOutputRate(timing)).toBe(50)
+    expect(getDisplayOutputRate(timing)).toBe(50)
+  })
+
+  it('does not calculate output rate without output tokens', () => {
+    expect(calculateOutputRate({
+      output_tokens: 0,
+      response_time_ms: 1000,
+      first_byte_time_ms: 500,
+    })).toBeNull()
+  })
+
+  it('does not calculate output rate when first byte is not before completion', () => {
+    expect(calculateOutputRate({
+      output_tokens: 50,
+      response_time_ms: 1000,
+      first_byte_time_ms: 1000,
+      upstream_is_stream: true,
+    })).toBeNull()
+  })
+
+  it('hides implausible rates from very short generation tails', () => {
+    const timing = {
+      output_tokens: 300,
+      response_time_ms: 1000,
+      first_byte_time_ms: 950,
+      upstream_is_stream: true,
+    }
+    const rate = calculateOutputRate(timing)
+
+    expect(rate).toBe(6000)
+    expect(shouldHideOutputRate(rate, timing)).toBe(true)
+    expect(getDisplayOutputRate(timing)).toBeNull()
+  })
+
+  it('keeps normal output rates visible', () => {
+    const timing = {
+      output_tokens: 50,
+      response_time_ms: 1000,
+      first_byte_time_ms: 500,
+      upstream_is_stream: true,
+    }
+
+    expect(getDisplayOutputRate(timing)).toBe(100)
+  })
+
+  it('calculates standard output rate without first byte timing', () => {
+    expect(getDisplayOutputRate({
+      output_tokens: 50,
+      response_time_ms: 1000,
+      upstream_is_stream: false,
+    })).toBe(50)
+  })
+
+  it('formats durations and output rates for compact UI display', () => {
+    expect(formatDurationMs(456)).toBe('456ms')
+    expect(formatDurationMs(1234)).toBe('1.23s')
+    expect(formatOutputRateValue(87.65)).toBe('87.7')
+    expect(formatOutputRate(87.65)).toBe('87.7 tps')
+    expect(formatOutputRate(1200)).toBe('1,200 tps')
+  })
+})