Queue cancellation mechanism:
- Add CancelByAlertIDs method to mark queued notifications as cancelled when alerts resolve
- Update CancelAlert to cancel queued notifications containing resolved alert IDs
- Skip cancelled notifications in queue processor
- Prevents resolved alerts from triggering notifications after they clear
Atomic DB operations:
- Add IncrementAttemptAndSetStatus to atomically update attempt counter and status
- Replace separate IncrementAttempt + UpdateStatus calls with single atomic operation
- Prevents orphaned queue entries when crashes occur between operations
- Eliminates race condition where rows get stuck in "pending" or "sending" status
These fixes ensure queued notifications are properly cancelled when alerts resolve
and prevent database inconsistencies during crash scenarios.
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