Save notification content to storage immediately after scheduling
rollover notification so it can be retrieved when the notification
fires. Without this, processRollover fails to find the content
and cannot schedule the next notification.
The rollover flow creates a new notification with a new ID
(daily_rollover_*) but was only scheduling it with the system,
not saving it to storage. When the notification fired, the
lookup by ID failed because the content wasn't stored.
This matches the pattern used in DailyNotificationScheduleHelper
which saves content before scheduling.
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