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>

backend/internal/service/scheduled_test_service.go

@@ -0,0 +1,102 @@
+package service
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/robfig/cron/v3"
+)
+
+var scheduledTestCronParser = cron.NewParser(cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow)
+
+// ScheduledTestService provides CRUD operations for scheduled test plans and results.
+type ScheduledTestService struct {
+	planRepo   ScheduledTestPlanRepository
+	resultRepo ScheduledTestResultRepository
+}
+
+// NewScheduledTestService creates a new ScheduledTestService.
+func NewScheduledTestService(
+	planRepo ScheduledTestPlanRepository,
+	resultRepo ScheduledTestResultRepository,
+) *ScheduledTestService {
+	return &ScheduledTestService{
+		planRepo:   planRepo,
+		resultRepo: resultRepo,
+	}
+}
+
+// CreatePlan validates the cron expression, computes next_run_at, and persists the plan.
+func (s *ScheduledTestService) CreatePlan(ctx context.Context, plan *ScheduledTestPlan) (*ScheduledTestPlan, error) {
+	nextRun, err := computeNextRun(plan.CronExpression, time.Now())
+	if err != nil {
+		return nil, fmt.Errorf("invalid cron expression: %w", err)
+	}
+	plan.NextRunAt = &nextRun
+
+	if plan.MaxResults <= 0 {
+		plan.MaxResults = 50
+	}
+
+	return s.planRepo.Create(ctx, plan)
+}
+
+// GetPlan retrieves a plan by ID.
+func (s *ScheduledTestService) GetPlan(ctx context.Context, id int64) (*ScheduledTestPlan, error) {
+	return s.planRepo.GetByID(ctx, id)
+}
+
+// ListPlansByAccount returns all plans for a given account.
+func (s *ScheduledTestService) ListPlansByAccount(ctx context.Context, accountID int64) ([]*ScheduledTestPlan, error) {
+	return s.planRepo.ListByAccountID(ctx, accountID)
+}
+
+// UpdatePlan validates cron and updates the plan.
+func (s *ScheduledTestService) UpdatePlan(ctx context.Context, plan *ScheduledTestPlan) (*ScheduledTestPlan, error) {
+	nextRun, err := computeNextRun(plan.CronExpression, time.Now())
+	if err != nil {
+		return nil, fmt.Errorf("invalid cron expression: %w", err)
+	}
+	plan.NextRunAt = &nextRun
+
+	return s.planRepo.Update(ctx, plan)
+}
+
+// DeletePlan removes a plan and its results (via CASCADE).
+func (s *ScheduledTestService) DeletePlan(ctx context.Context, id int64) error {
+	return s.planRepo.Delete(ctx, id)
+}
+
+// ListResults returns the most recent results for a plan.
+func (s *ScheduledTestService) ListResults(ctx context.Context, planID int64, limit int) ([]*ScheduledTestResult, error) {
+	if limit <= 0 {
+		limit = 50
+	}
+	return s.resultRepo.ListByPlanID(ctx, planID, limit)
+}
+
+// SaveResult inserts a result and prunes old entries beyond maxResults.
+func (s *ScheduledTestService) SaveResult(ctx context.Context, planID int64, maxResults int, outcome *ScheduledTestOutcome) error {
+	result := &ScheduledTestResult{
+		PlanID:       planID,
+		Status:       outcome.Status,
+		ResponseText: outcome.ResponseText,
+		ErrorMessage: outcome.ErrorMessage,
+		LatencyMs:    outcome.LatencyMs,
+		StartedAt:    outcome.StartedAt,
+		FinishedAt:   outcome.FinishedAt,
+	}
+	if _, err := s.resultRepo.Create(ctx, result); err != nil {
+		return err
+	}
+	return s.resultRepo.PruneOldResults(ctx, planID, maxResults)
+}
+
+func computeNextRun(cronExpr string, from time.Time) (time.Time, error) {
+	sched, err := scheduledTestCronParser.Parse(cronExpr)
+	if err != nil {
+		return time.Time{}, err
+	}
+	return sched.Next(from), nil
+}