Introduce a comprehensive alert management system for monitoring
system events and notifications.
Changes include:
- Add Alert model with type, severity, status, and metadata fields
- Implement AlertHandler with full CRUD operations (create, list,
get, acknowledge, resolve, dismiss)
- Add alert statistics endpoint for counts by status and severity
- Register Alert model in database auto-migration
- Add minute-level aggregation to log stats (limited to 6-hour range)
