feat(bot): add kawaii point reminder DMs

[nathan2slime]

Summary

• add a background reminder loop that resolves the Discord application owner and sends DM reminders at 12:00, 13:05, and 17:53 local time
• make the reminder message kawaii-themed with rotating anime GIFs while ensuring the task starts only once across gateway reconnects
• document the new reminder capability and cover the reminder scheduling logic with tests

Testing

• cargo fmt --all
• cargo test