Fix Telegram notification spam with channel-level rate limiting (#951)

* Fix Telegram notification spam with channel-level rate limiting

Addresses issue #950 where Telegram notifications were sent excessively
(96-288 messages per day per task) due to the delete-and-recreate pattern
added in commit 105a913a to fix navbar notification pile-up.

Changes:
- Add channel_sent_at JSON field to notifications table to track when
  each channel (telegram, email, push) was last sent
- Add helper methods to notification model:
  - markChannelAsSent(channel): Records send timestamp
  - wasChannelRecentlySent(channel, threshold): Checks if sent within 24h
- Modify sendTelegramNotification() to check rate limit before sending
- Update service layer (dueTaskService, deferredTaskService,
  dueProjectService) to preserve channel_sent_at when recreating
  notifications
- Add comprehensive unit and integration tests (20 tests, all passing)

Impact:
- Reduces Telegram notifications from 96-288/day to 1/day per item
- Preserves in-app notification refresh behavior (every 5-15 min)
- Maintains navbar pile-up fix from original commit
- Rate limit configurable (default: 24 hours)

Fixes #950

* Fix linting and formatting issues

* Fix integration test that was trying to access private function

* Fix prettier formatting in integration test

backend/modules/tasks/deferredTaskService.js

@@ -67,6 +67,9 @@ async function checkDeferredTasks() {
                         notif.data?.reason === 'defer_until_reached'
                 );
 
+                // Preserve channel_sent_at for rate limiting when recreating notifications
+                let preservedChannelSentAt = null;
+
                 if (existingNotification) {
                     // If notification was dismissed, don't create it again
                     if (existingNotification.dismissed_at) {
@@ -76,6 +79,9 @@ async function checkDeferredTasks() {
                     // If notification is unread, delete it before creating the new one
                     // This prevents duplicate notifications from piling up
                     if (!existingNotification.read_at) {
+                        // Preserve channel_sent_at to maintain rate limiting across recreations
+                        preservedChannelSentAt =
+                            existingNotification.channel_sent_at;
                         await existingNotification.destroy();
                     } else {
                         // If it was already read, skip creating a new one
@@ -101,6 +107,7 @@ async function checkDeferredTasks() {
                         reason: 'defer_until_reached',
                     },
                     sentAt: new Date(),
+                    channel_sent_at: preservedChannelSentAt,
                 });
 
                 notificationsCreated++;