Fix notification manager deadlock in Stop()

Critical deadlock fix:
- Stop() was holding n.mu lock while calling queue.Stop()
- queue.Stop() waits for worker goroutines to finish
- Worker goroutines call ProcessQueuedNotification() which needs n.mu lock
- This created a classic lock-order deadlock

Fix:
- Unlock n.mu before calling queue.Stop()
- Relock after queue shutdown completes
- Workers can now finish and acquire lock as needed

This resolves 30-second test timeouts in notifications package.

Tests now complete in <1s instead of timing out at 30s.

internal/notifications/notifications.go

@@ -2537,16 +2537,26 @@ func (n *NotificationManager) cleanupOldNotificationRecords() {
 // Stop gracefully stops the notification manager
 func (n *NotificationManager) Stop() {
 	n.mu.Lock()
-	defer n.mu.Unlock()
 
 	// Stop cleanup goroutine
 	close(n.stopCleanup)
 
+	// Get queue reference before unlocking
+	queue := n.queue
+
+	// Unlock before stopping queue to avoid deadlock with queue workers
+	// that may need to acquire n.mu during ProcessQueuedNotification
+	n.mu.Unlock()
+
 	// Stop the notification queue if it exists
-	if n.queue != nil {
-		n.queue.Stop()
+	if queue != nil {
+		queue.Stop()
 	}
 
+	// Relock for remaining cleanup
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
 	// Cancel any pending group timer
 	if n.groupTimer != nil {
 		n.groupTimer.Stop()