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/models/notification.go

@@ -20,6 +20,9 @@ type NotificationPreference struct {
 
 	// Document notifications
 	WarrantyExpiring bool `gorm:"column:warranty_expiring;default:true" json:"warranty_expiring"`
+
+	// Email preferences
+	EmailTaskCompleted bool `gorm:"column:email_task_completed;default:true" json:"email_task_completed"`
 }
 
 // TableName returns the table name for GORM