feat: 支持基于 crontab 的定时账号测试

每个测试计划绑定一个账号和一个模型，按 cron 表达式定期执行测试，
保存历史结果并在前端账号管理页面中提供完整的增删改查和结果查看功能。

主要变更：
- 新增 scheduled_test_plans / scheduled_test_results 两张表及迁移
- 后端 service 层：CRUD 服务 + 后台 cron runner（每分钟扫描到期计划并发执行）
- RunTestBackground 方法通过 httptest 在内存中执行账号测试并解析 SSE 输出
- Redis leader lock + pg_try_advisory_lock 双重保障多实例部署只执行一次
- REST API：5 个管理端点（计划 CRUD + 结果查询）
- 前端 ScheduledTestsPanel 组件：计划管理、启用开关、内联编辑、结果展开查看
- 中英文 i18n 支持

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/i18n/locales/en.ts

@@ -2413,6 +2413,34 @@ export default {
         'This account is not eligible for Antigravity, but API forwarding still works. Use at your own risk.'
     },
 
+    // Scheduled Tests
+    scheduledTests: {
+      title: 'Scheduled Tests',
+      addPlan: 'Add Plan',
+      editPlan: 'Edit Plan',
+      deletePlan: 'Delete Plan',
+      model: 'Model',
+      cronExpression: 'Cron Expression',
+      enabled: 'Enabled',
+      lastRun: 'Last Run',
+      nextRun: 'Next Run',
+      maxResults: 'Max Results',
+      noPlans: 'No scheduled test plans',
+      confirmDelete: 'Are you sure you want to delete this plan?',
+      createSuccess: 'Plan created successfully',
+      updateSuccess: 'Plan updated successfully',
+      deleteSuccess: 'Plan deleted successfully',
+      results: 'Test Results',
+      noResults: 'No test results yet',
+      responseText: 'Response',
+      errorMessage: 'Error',
+      success: 'Success',
+      failed: 'Failed',
+      running: 'Running',
+      schedule: 'Schedule',
+      cronHelp: 'Standard 5-field cron expression (e.g., */30 * * * *)'
+    },
+
     // Proxies
     proxies: {
       title: 'Proxy Management',