Change notification ID type from int to string (CHAR(12)) to match the schema migration.
- Update AdminNotification.ID field to string
- Change cursor format from int-based to string-based for pagination
- Use validators.IsValidID instead of strconv.Atoi for notification ID validation
- Update cursor format to use RFC3339Nano for precision
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
- Add AcknowledgePendingBookingNotification helper for acknowledging notifications
- ConfirmBookingHandler: acknowledge pending notification when booking confirmed
- Cancel handlers: acknowledge pending notification and only create cancelled_booking notification if booking was not in pending status
- Add user_notification_preferences table with email, sms, push enabled flags
- Update cancellation logic to check original status before creating notifications