Implement Scheduled Security Test Execution

## Overview
Add the ability to schedule security tests to run automatically on a recurring basis.

## Business Value
Automated scheduled testing ensures continuous security monitoring without manual intervention.

## Current State
- Security testing framework exists
- Tests can be run manually
- No scheduling system

## Subtasks
- [ ] Design scheduling system
- [ ] Implement cron-based scheduling
- [ ] Create test schedule database schema
- [ ] Add schedule management API
- [ ] Implement schedule UI
- [ ] Add schedule notifications
- [ ] Create schedule execution engine

## Implementation Steps
1. Create backend/internal/security/scheduler.go
2. Add scheduling database tables
3. Implement cron parser
4. Create schedule management endpoints
5. Build schedule UI component

## Acceptance Criteria
- [ ] Tests can be scheduled with cron expressions
- [ ] Schedules are stored and persisted
- [ ] Scheduled tests execute automatically
- [ ] Schedule results are stored
- [ ] Users can manage schedules via UI
- [ ] Notifications sent on schedule completion

## Estimated Effort
- Hours: 16-20
- Complexity: High

Start by reading the existing security testing framework and the proposed backend/internal/security/scheduler.go entry point. Trace how manual test execution currently works, then review the database, management API, UI, execution, persistence, and notification subtasks. Done means recurring cron schedules can be managed, persisted, executed, and reported through the stated acceptance criteria.

由索引模型根据 Issue 内容生成。