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/i18n/locales/en.ts

@@ -722,8 +722,15 @@ export default {
     unknown: 'Unknown',
     in: 'In',
     out: 'Out',
+    inputTokenPrice: 'Input price',
+    outputTokenPrice: 'Output price',
+    perMillionTokens: '/ 1M tokens',
     cacheRead: 'Read',
     cacheWrite: 'Write',
+    serviceTier: 'Service tier',
+    serviceTierPriority: 'Fast',
+    serviceTierFlex: 'Flex',
+    serviceTierStandard: 'Standard',
     rate: 'Rate',
     original: 'Original',
     billed: 'Billed',