fix: change register verify_code field to code to match frontend

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/model/user/user.go

@@ -32,7 +32,7 @@ type EmailVerifyCodeResponse struct{}
 type EmailRegisterRequest struct {
 	Email      string `json:"email" binding:"required,email"`
 	Password   string `json:"password" binding:"required,min=6"`
-	VerifyCode string `json:"verify_code" binding:"required"`
+	VerifyCode string `json:"code" binding:"required"`
 }
 
 type EmailRegisterResponse struct {