Enable notifications by default for new and existing users

[tiankaima]

Summary

• Turn on class, homework, Young Community, and todo reminders by default for new users when they log in.
• Upgrade the current production SQLite database from version 5 to 6 once, enabling all four reminders for existing users and creating settings for users without a row. Users without credentials, or already paused for reauthorization, remain paused until login.
• Preserve each user's later opt-out across token refreshes and future deployments.

Verification

• go test ./...
• make check-scripts
• Production database inspected read-only: version 5, 155 users, 24 credentials, 2 existing notification settings rows.

The production Bot still logs intermittent 401 responses from Young Community notifications and 503 responses from homework notifications. This change does not alter those upstream responses.