feat: 新增全局错误透传规则功能

支持管理员配置上游错误如何返回给客户端：
- 新增 ErrorPassthroughRule 数据模型和 Ent Schema
- 实现规则的 CRUD API（/admin/error-passthrough-rules）
- 支持按错误码、关键词匹配，支持 any/all 匹配模式
- 支持按平台过滤（anthropic/openai/gemini/antigravity）
- 支持透传或自定义响应状态码和错误消息
- 实现两级缓存（Redis + 本地内存）和多实例同步
- 集成到 gateway_handler 的错误处理流程
- 新增前端管理界面组件
- 新增单元测试覆盖核心匹配逻辑

优化：
- 移除 refreshLocalCache 中的冗余排序（数据库已排序）
- 后端 Validate() 增加匹配条件非空校验

frontend/src/i18n/locales/en.ts

@@ -3191,6 +3191,80 @@ export default {
       failedToSave: 'Failed to save settings',
       failedToTestSmtp: 'SMTP connection test failed',
       failedToSendTestEmail: 'Failed to send test email'
+    },
+
+    // Error Passthrough Rules
+    errorPassthrough: {
+      title: 'Error Passthrough Rules',
+      description: 'Configure how upstream errors are returned to clients',
+      createRule: 'Create Rule',
+      editRule: 'Edit Rule',
+      deleteRule: 'Delete Rule',
+      noRules: 'No rules configured',
+      createFirstRule: 'Create your first error passthrough rule',
+      allPlatforms: 'All Platforms',
+      passthrough: 'Passthrough',
+      custom: 'Custom',
+      code: 'Code',
+      body: 'Body',
+
+      // Columns
+      columns: {
+        priority: 'Priority',
+        name: 'Name',
+        conditions: 'Conditions',
+        platforms: 'Platforms',
+        behavior: 'Behavior',
+        status: 'Status',
+        actions: 'Actions'
+      },
+
+      // Match Mode
+      matchMode: {
+        any: 'Code OR Keyword',
+        all: 'Code AND Keyword',
+        anyHint: 'Status code matches any error code, OR message contains any keyword',
+        allHint: 'Status code matches any error code, AND message contains any keyword'
+      },
+
+      // Form
+      form: {
+        name: 'Rule Name',
+        namePlaceholder: 'e.g., Context Limit Passthrough',
+        priority: 'Priority',
+        priorityHint: 'Lower values have higher priority',
+        description: 'Description',
+        descriptionPlaceholder: 'Describe the purpose of this rule...',
+        matchConditions: 'Match Conditions',
+        errorCodes: 'Error Codes',
+        errorCodesPlaceholder: '422, 400, 429',
+        errorCodesHint: 'Separate multiple codes with commas',
+        keywords: 'Keywords',
+        keywordsPlaceholder: 'One keyword per line\ncontext limit\nmodel not supported',
+        keywordsHint: 'One keyword per line, case-insensitive',
+        matchMode: 'Match Mode',
+        platforms: 'Platforms',
+        platformsHint: 'Leave empty to apply to all platforms',
+        responseBehavior: 'Response Behavior',
+        passthroughCode: 'Passthrough upstream status code',
+        responseCode: 'Custom status code',
+        passthroughBody: 'Passthrough upstream error message',
+        customMessage: 'Custom error message',
+        customMessagePlaceholder: 'Error message to return to client...',
+        enabled: 'Enable this rule'
+      },
+
+      // Messages
+      nameRequired: 'Please enter rule name',
+      conditionsRequired: 'Please configure at least one error code or keyword',
+      ruleCreated: 'Rule created successfully',
+      ruleUpdated: 'Rule updated successfully',
+      ruleDeleted: 'Rule deleted successfully',
+      deleteConfirm: 'Are you sure you want to delete rule "{name}"?',
+      failedToLoad: 'Failed to load rules',
+      failedToSave: 'Failed to save rule',
+      failedToDelete: 'Failed to delete rule',
+      failedToToggle: 'Failed to toggle status'
     }
   },