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_passkey.go

@@ -0,0 +1,40 @@
+package repo
+
+import (
+	"errors"
+
+	"go-backend/internal/store/model"
+	"gorm.io/gorm"
+)
+
+func (r *Repository) ListPasskeys(userID int64) ([]model.Passkey, error) {
+	var keys []model.Passkey
+	err := r.db.Where("user_id = ?", userID).Order("created_at desc").Find(&keys).Error
+	return keys, err
+}
+
+func (r *Repository) GetPasskey(userID int64, id string) (*model.Passkey, error) {
+	var key model.Passkey
+	err := r.db.Where("user_id = ? AND id = ?", userID, id).Take(&key).Error
+	if errors.Is(err, gorm.ErrRecordNotFound) {
+		return nil, nil
+	}
+	return &key, err
+}
+
+func (r *Repository) CreatePasskey(key *model.Passkey) error {
+	return r.db.Create(key).Error
+}
+
+func (r *Repository) DeletePasskey(userID int64, id string) (bool, error) {
+	result := r.db.Where("user_id = ? AND id = ?", userID, id).Delete(&model.Passkey{})
+	return result.RowsAffected == 1, result.Error
+}
+
+// Compare-and-swap prevents a concurrent assertion from reusing an old sign counter.
+func (r *Repository) UpdatePasskeyCredential(userID int64, id, oldJSON, newJSON string, now int64) (bool, error) {
+	result := r.db.Model(&model.Passkey{}).
+		Where("user_id = ? AND id = ? AND credential_json = ?", userID, id, oldJSON).
+		Updates(map[string]interface{}{"credential_json": newJSON, "last_used_at": now})
+	return result.RowsAffected == 1, result.Error
+}