fix(user): add VerificationStatusNone for users without verification

Add VerificationStatusNone constant and update converter to return
general identity with none status instead of empty strings when user
has not applied for verification.

internal/model/user.go

@@ -54,6 +54,7 @@ const (
 	VerificationStatusPending      VerificationStatus = "pending"       // 审核中
 	VerificationStatusApproved     VerificationStatus = "approved"      // 已通过
 	VerificationStatusRejected     VerificationStatus = "rejected"      // 已拒绝
+	VerificationStatusNone         VerificationStatus = "none"          // 无认证（对外展示用）
 )
 
 // User 用户实体