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/usagePricing.ts

@@ -0,0 +1,49 @@
+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}`
+}