fix(billing): 修复 OpenAI fast 档位计费并补齐展示

- 打通 service_tier 在 OpenAI HTTP、WS、passthrough 与 usage 记录中的传递
- 修正 priority/flex 计费逻辑，并将 fast 归一化为 priority
- 在用户端和管理端补齐服务档位与计费明细展示
- 补齐前后端测试，并修复 WS 限流信号重复持久化导致的全量回归失败

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/utils/__tests__/usageServiceTier.spec.ts

@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest'
+
+import { formatUsageServiceTier, getUsageServiceTierLabel, normalizeUsageServiceTier } from '@/utils/usageServiceTier'
+
+describe('usageServiceTier utils', () => {
+  it('normalizes fast/default aliases', () => {
+    expect(normalizeUsageServiceTier('fast')).toBe('priority')
+    expect(normalizeUsageServiceTier(' default ')).toBe('standard')
+    expect(normalizeUsageServiceTier('STANDARD')).toBe('standard')
+  })
+
+  it('preserves supported tiers', () => {
+    expect(normalizeUsageServiceTier('priority')).toBe('priority')
+    expect(normalizeUsageServiceTier('flex')).toBe('flex')
+  })
+
+  it('formats empty values as standard', () => {
+    expect(formatUsageServiceTier()).toBe('standard')
+    expect(formatUsageServiceTier('')).toBe('standard')
+  })
+
+  it('passes through unknown non-empty tiers for display fallback', () => {
+    expect(normalizeUsageServiceTier('custom-tier')).toBe('custom-tier')
+    expect(formatUsageServiceTier('custom-tier')).toBe('custom-tier')
+  })
+
+  it('maps tiers to translated labels', () => {
+    const translate = (key: string) => ({
+      'usage.serviceTierPriority': 'Fast',
+      'usage.serviceTierFlex': 'Flex',
+      'usage.serviceTierStandard': 'Standard',
+    })[key] ?? key
+
+    expect(getUsageServiceTierLabel('fast', translate)).toBe('Fast')
+    expect(getUsageServiceTierLabel('flex', translate)).toBe('Flex')
+    expect(getUsageServiceTierLabel(undefined, translate)).toBe('Standard')
+    expect(getUsageServiceTierLabel('custom-tier', translate)).toBe('custom-tier')
+  })
+})

frontend/src/utils/usagePricing.ts

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+export const TOKENS_PER_MILLION = 1_000_000
+
+interface TokenPriceFormatOptions {
+  fractionDigits?: number
+  withCurrencySymbol?: boolean
+  emptyValue?: string
+}
+
+function isFiniteNumber(value: unknown): value is number {
+  return typeof value === 'number' && Number.isFinite(value)
+}
+
+export function calculateTokenUnitPrice(
+  cost: number | null | undefined,
+  tokens: number | null | undefined
+): number | null {
+  if (!isFiniteNumber(cost) || !isFiniteNumber(tokens) || tokens <= 0) {
+    return null
+  }
+
+  return cost / tokens
+}
+
+export function calculateTokenPricePerMillion(
+  cost: number | null | undefined,
+  tokens: number | null | undefined
+): number | null {
+  const unitPrice = calculateTokenUnitPrice(cost, tokens)
+  if (unitPrice == null) {
+    return null
+  }
+
+  return unitPrice * TOKENS_PER_MILLION
+}
+
+export function formatTokenPricePerMillion(
+  cost: number | null | undefined,
+  tokens: number | null | undefined,
+  options: TokenPriceFormatOptions = {}
+): string {
+  const pricePerMillion = calculateTokenPricePerMillion(cost, tokens)
+  if (pricePerMillion == null) {
+    return options.emptyValue ?? '-'
+  }
+
+  const fractionDigits = options.fractionDigits ?? 4
+  const formatted = pricePerMillion.toFixed(fractionDigits)
+  return options.withCurrencySymbol == false ? formatted : `$${formatted}`
+}

frontend/src/utils/usageServiceTier.ts

@@ -0,0 +1,25 @@
+export function normalizeUsageServiceTier(serviceTier?: string | null): string | null {
+  const value = serviceTier?.trim().toLowerCase()
+  if (!value) return null
+  if (value === 'fast') return 'priority'
+  if (value === 'default' || value === 'standard') return 'standard'
+  if (value === 'priority' || value === 'flex') return value
+  return value
+}
+
+export function formatUsageServiceTier(serviceTier?: string | null): string {
+  const normalized = normalizeUsageServiceTier(serviceTier)
+  if (!normalized) return 'standard'
+  return normalized
+}
+
+export function getUsageServiceTierLabel(
+  serviceTier: string | null | undefined,
+  translate: (key: string) => string,
+): string {
+  const tier = formatUsageServiceTier(serviceTier)
+  if (tier === 'priority') return translate('usage.serviceTierPriority')
+  if (tier === 'flex') return translate('usage.serviceTierFlex')
+  if (tier === 'standard') return translate('usage.serviceTierStandard')
+  return tier
+}