This commit resolves several issues with the notification scheduler:
- DMs are no longer delayed, and notifications are sent at the correct times.
- The lesson selection logic now correctly identifies the next lesson.
- The Discord webhook now sends notifications reliably.
A new, robust scheduler has been implemented to prevent missed notifications due to timing drift, and the `Run` function has been refactored to ensure the correct lesson is always selected. Additionally, a check has been added to prevent silent webhook failures when the URL is not configured.
