Implement subscription SLA monitoring with breach alerts

## Context
SLAs need monitoring with breach alerts.

## Current Limitation
No SLA monitoring exists.

## Expected Outcome
- SLA tracking
- Breach detection
- SLA alerts
- SLA reporting

## Acceptance Criteria
- [ ] SLA definition per tier
- [ ] Real-time SLA tracking
- [ ] Breach detection and alerts
- [ ] SLA reporting
- [ ] SLA analytics
- [ ] SLA credits
- [ ] SLA documentation