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