feat: TimeOff reply cooldown

[scuciatto]

Summary

• Adds a configurable cooldown period to prevent the app from sending repeated TimeOff notifications to the same sender within a set time window
• Introduces a new app setting (timeoff_reply_cooldown_hours, default: 12h) configurable via the Rocket.Chat admin panel
• Tracks per-sender last-notified timestamps on the ITimeOff record and cleans up expired entries automatically

Test plan

• Set a time-off message and send a message from another user — confirm the notification fires
• Send another message from the same user within the cooldown window — confirm no second notification is sent
• Wait for the cooldown to expire (or set it to 0) — confirm the notification fires again
• Change the cooldown setting in the admin panel and verify the new value is respected
• Verify the app starts cleanly and the new setting appears in the admin panel