- Add monitoring.py: CostSnapshot, CronJobStatus, SessionEvent, SubAgentRun, SystemHealthMetric
- Add alert_rules.py: AlertRule, AlertEvent
- Register all 7 models in __init__.py
- Add Alembic migration 7a8b9c0d1e2f for 7 new monitoring tables
- Add Alembic migration 8f9a0b1c2d3e for CASCADE FK rules, composite indexes, acknowledged_by FK
- Update env.py for transaction-per-migration to avoid failure chaining
- Security: ondelete CASCADE on all org/gateway FKs, SET NULL on acknowledged_by
- Performance: composite indexes on (org_id, created_at) and (org_id, gateway_id) for all monitoring tables
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