feat: support secure passkey login (#561)

Add WebAuthn passkey enrollment and login for issue #536, with trusted origin configuration, user verification, single-use challenges, and documented deployment and recovery behavior.

go-backend/internal/store/repo/repository_mutations.go

@@ -156,6 +156,9 @@ func (r *Repository) DeleteUserCascade(userID int64) error {
 		return errors.New("repository not initialized")
 	}
 	return r.db.Transaction(func(tx *gorm.DB) error {
+		if err := tx.Where("user_id = ?", userID).Delete(&model.Passkey{}).Error; err != nil {
+			return err
+		}
 		forwardIDs := tx.Model(&model.Forward{}).Select("id").Where("user_id = ?", userID)
 		if err := tx.Where("forward_id IN (?)", forwardIDs).Delete(&model.ForwardPort{}).Error; err != nil {
 			return err