Add email notification preference for task completion

- Add EmailTaskCompleted field to NotificationPreference model
- Update notification repository to include email preference in queries
- Check email preference before sending task completion emails
- Add email preference toggle to admin panel notification-prefs page
- Update API types for email preference support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/repositories/notification_repo.go

@@ -142,13 +142,14 @@ func (r *NotificationRepository) GetOrCreatePreferences(userID uint) (*models.No
 
 	if err == gorm.ErrRecordNotFound {
 		prefs = &models.NotificationPreference{
-			UserID:           userID,
-			TaskDueSoon:      true,
-			TaskOverdue:      true,
-			TaskCompleted:    true,
-			TaskAssigned:     true,
-			ResidenceShared:  true,
-			WarrantyExpiring: true,
+			UserID:             userID,
+			TaskDueSoon:        true,
+			TaskOverdue:        true,
+			TaskCompleted:      true,
+			TaskAssigned:       true,
+			ResidenceShared:    true,
+			WarrantyExpiring:   true,
+			EmailTaskCompleted: true,
 		}
 		if err := r.CreatePreferences(prefs); err != nil {
 			return nil, err