feat(anthropic): 支持 API Key 自动透传并优化透传链路性能

- 新增 Anthropic API Key 自动透传开关与后端透传分支（仅替换认证）

- 账号编辑页新增自动透传开关，默认关闭

- 优化透传性能：SSE usage 解析 gjson 快路径、减少请求体重复拷贝、优化流式写回与非流式 usage 解析

- 补充单元测试与 benchmark，确保 Claude OAuth 路径不受影响

frontend/src/i18n/locales/en.ts

@@ -1546,6 +1546,11 @@ export default {
         enableSora: 'Enable Sora simultaneously',
         enableSoraHint: 'Sora uses the same OpenAI account. Enable to create Sora account simultaneously.'
       },
+      anthropic: {
+        apiKeyPassthrough: 'Auto passthrough (auth only)',
+        apiKeyPassthroughDesc:
+          'Only applies to Anthropic API Key accounts. When enabled, messages/count_tokens are forwarded in passthrough mode with auth replacement only, while billing/concurrency/audit and safety filtering are preserved. Disable to roll back immediately.'
+      },
       modelRestriction: 'Model Restriction (Optional)',
       modelWhitelist: 'Model Whitelist',
       modelMapping: 'Model Mapping',