Implement 5 medium/low priority improvements identified in systematic review:
UX IMPROVEMENTS:
- Notify existing critical alerts when activating from pending_review state
Previously: critical alerts during observation window would never notify
Now: users receive notifications for active critical alerts after activation
Implementation: Added NotifyExistingAlert() method and logic in ActivateAlerts()
PERFORMANCE OPTIMIZATIONS:
- Replace per-alert cleanup goroutines with periodic batch cleanup
Prevents spawning 1000s of goroutines during alert flapping
recentlyResolved entries now cleaned up once per minute instead of 1 goroutine per alert
- Simplify GetActiveAlerts() implementation
Removed intermediate map copy, holds lock slightly longer but operation is fast
Cleaner code with reduced memory allocation
CONFIGURATION VALIDATION:
- Validate timezone in quiet hours configuration
Invalid timezones now disable quiet hours with error log instead of silent fallback
Prevents unexpected behavior when timezone is typo'd or invalid
GRACEFUL SHUTDOWN:
- Add 100ms delay in Stop() for background goroutine cleanup
Reduces risk of state corruption during shutdown
Allows escalation checker and periodic save to exit cleanly
Technical details:
- internal/alerts/alerts.go: Added NotifyExistingAlert(), optimized cleanup patterns
- internal/api/alerts.go: Enhanced ActivateAlerts() to notify existing critical alerts
- Removed ~20 lines of goroutine spawning code
- Added periodic cleanup for recentlyResolved map
- All changes preserve backward compatibility
Testing: Verified compilation with 'go build -o /dev/null ./...'
