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/__tests__/usageServiceTierLocales.spec.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest'
+
+import en from '../locales/en'
+import zh from '../locales/zh'
+
+describe('usage service tier locale keys', () => {
+  it('contains zh labels for service tier tooltip', () => {
+    expect(zh.usage.serviceTier).toBe('服务档位')
+    expect(zh.usage.serviceTierPriority).toBe('Fast')
+    expect(zh.usage.serviceTierFlex).toBe('Flex')
+    expect(zh.usage.serviceTierStandard).toBe('Standard')
+  })
+
+  it('contains en labels for service tier tooltip', () => {
+    expect(en.usage.serviceTier).toBe('Service tier')
+    expect(en.usage.serviceTierPriority).toBe('Fast')
+    expect(en.usage.serviceTierFlex).toBe('Flex')
+    expect(en.usage.serviceTierStandard).toBe('Standard')
+  })
+})