Security Enhancement: Add Authentication to Subscription Manager

The subscription manager currently implements direct subscribe/unsubscribe without email confirmation due to email delivery limitations. This creates a security concern where anyone can potentially:
- Subscribe arbitrary @inspection.gc.ca email addresses
- Unsubscribe others from the mailing list

## Current Implementation

After removing email confirmation (due to SMTP constraints), the application now allows:
- Direct subscription by entering any @inspection.gc.ca email
- Direct unsubscription by entering any subscribed email
- No identity verification of the person making the request

## Security Risks

1. **Unauthorized Subscriptions**: Malicious actors could subscribe colleagues without consent
2. **Unauthorized Unsubscriptions**: Anyone could remove legitimate subscribers
3. **No Audit Trail**: Limited logging of who performed subscription actions

## Proposed Solutions (Future Enhancements)

### Option 1: CFIA Authentication Integration
- Integrate with existing CFIA authentication system (SAML/SSO)
- Verify user identity before allowing subscription changes
- Maintain audit trail of authenticated actions

### Option 2: Admin-Only Management
- Remove public subscription interface
- Implement admin panel for subscription management
- Require admin credentials for any changes

### Option 3: Enhanced Verification
- Add additional verification fields (employee ID, department)
- Implement rate limiting and IP restrictions
- Add CAPTCHA to prevent automated abuse

### Option 4: Hybrid Approach
- Allow self-subscription with enhanced verification
- Require admin approval for unsubscriptions
- Email notifications to admins for all changes

## Implementation Priority

This should be addressed before production deployment to ensure:
- User privacy protection
- Prevention of subscription abuse
- Compliance with internal security policies

## Additional Considerations

- Email normalization (convert to lowercase) has been implemented
- Redis dependency could be removed if email confirmation is not restored
- Consider implementing request logging for security auditing