feat(gateway): 添加流超时处理机制

- 添加 StreamTimeoutSettings 配置结构体和系统设置
- 实现 TimeoutCounterCache Redis 计数器用于累计超时次数
- 在 RateLimitService 添加 HandleStreamTimeout 方法
- 在 gateway_service、openai_gateway_service、antigravity_gateway_service 中调用超时处理
- 添加后端 API 端点 GET/PUT /admin/settings/stream-timeout
- 添加前端配置界面到系统设置页面
- 支持配置：启用开关、超时阈值、处理方式、暂停时长、触发阈值、阈值窗口

默认配置：
- 启用：true
- 超时阈值：60秒
- 处理方式：临时不可调度
- 暂停时长：5分钟
- 触发阈值：3次
- 阈值窗口：10分钟

frontend/src/i18n/locales/en.ts

@@ -2512,6 +2512,27 @@ export default {
         securityWarning: 'Warning: This key provides full admin access. Keep it secure.',
         usage: 'Usage: Add to request header - x-api-key: <your-admin-api-key>'
       },
+      streamTimeout: {
+        title: 'Stream Timeout Handling',
+        description: 'Configure account handling strategy when upstream response times out',
+        enabled: 'Enable Stream Timeout Handling',
+        enabledHint: 'Automatically handle problematic accounts when upstream times out',
+        timeoutSeconds: 'Timeout Threshold (seconds)',
+        timeoutSecondsHint: 'Stream data interval exceeding this time is considered timeout (30-300s)',
+        action: 'Action',
+        actionTempUnsched: 'Temporarily Unschedulable',
+        actionError: 'Mark as Error',
+        actionNone: 'No Action',
+        actionHint: 'Action to take on the account after timeout',
+        tempUnschedMinutes: 'Pause Duration (minutes)',
+        tempUnschedMinutesHint: 'Duration of temporary unschedulable state (1-60 minutes)',
+        thresholdCount: 'Trigger Threshold (count)',
+        thresholdCountHint: 'Number of timeouts before triggering action (1-10)',
+        thresholdWindowMinutes: 'Threshold Window (minutes)',
+        thresholdWindowMinutesHint: 'Time window for counting timeouts (1-60 minutes)',
+        saved: 'Stream timeout settings saved',
+        saveFailed: 'Failed to save stream timeout settings'
+      },
       saveSettings: 'Save Settings',
       saving: 'Saving...',
       settingsSaved: 'Settings saved successfully',