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/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
+}