feat: 新增用户自助注册功能

后端新增 POST /api/v1/public/register 接口，支持门店自助注册并自动生成门店编码（S000001 格式）；shops 表加 description 字段。营销站新增 /register/ 注册页，含门店信息和管理员账号表单，注册成功后展示门店编码和登录提示。Flutter 登录页底部加「前往官网注册」跳转链接。

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

backend/internal/service/auth.go

@@ -2,9 +2,11 @@ package service
 
 import (
 	"errors"
+	"fmt"
 	"time"
 
 	"github.com/golang-jwt/jwt/v5"
+	"github.com/google/uuid"
 	"golang.org/x/crypto/bcrypt"
 	"gorm.io/gorm"
 
@@ -61,6 +63,76 @@ func (s *AuthService) Login(shopCode, username, password string) (*TokenPair, *m
 	return pair, &user, nil
 }
 
+// RegisterInput 注册新门店所需参数
+type RegisterInput struct {
+	ShopName    string `json:"shop_name"    binding:"required"`
+	Address     string `json:"address"      binding:"required"`
+	ManagerName string `json:"manager_name" binding:"required"`
+	Phone       string `json:"phone"        binding:"required"`
+	Description string `json:"description"`
+	Username    string `json:"username"     binding:"required"`
+	Password    string `json:"password"     binding:"required,min=6"`
+}
+
+// RegisterResult 注册成功后返回的数据
+type RegisterResult struct {
+	ShopCode string `json:"shop_code"`
+	ShopName string `json:"shop_name"`
+	Username string `json:"username"`
+}
+
+// Register 自助注册新门店（公开接口，无需认证）
+func (s *AuthService) Register(in RegisterInput) (*RegisterResult, error) {
+	hash, err := bcrypt.GenerateFromPassword([]byte(in.Password), bcrypt.DefaultCost)
+	if err != nil {
+		return nil, err
+	}
+
+	var result RegisterResult
+	err = s.db.Transaction(func(tx *gorm.DB) error {
+		// 先用 UUID 占位创建门店，得到真实 ID
+		shop := model.Shop{
+			Name:        in.ShopName,
+			Code:        uuid.New().String(),
+			Address:     in.Address,
+			Phone:       in.Phone,
+			ManagerName: in.ManagerName,
+			Description: in.Description,
+		}
+		if err := tx.Create(&shop).Error; err != nil {
+			return err
+		}
+
+		// 用自增 ID 生成正式门店编码
+		shop.Code = fmt.Sprintf("S%06d", shop.ID)
+		if err := tx.Model(&shop).Update("code", shop.Code).Error; err != nil {
+			return err
+		}
+
+		// 创建管理员用户
+		user := model.User{
+			ShopID:       shop.ID,
+			Username:     in.Username,
+			PasswordHash: string(hash),
+			RealName:     in.ManagerName,
+			Phone:        in.Phone,
+			Role:         "admin",
+			IsActive:     true,
+		}
+		if err := tx.Create(&user).Error; err != nil {
+			return err
+		}
+
+		result = RegisterResult{
+			ShopCode: shop.Code,
+			ShopName: shop.Name,
+			Username: user.Username,
+		}
+		return nil
+	})
+	return &result, err
+}
+
 // HashPassword 生成 bcrypt 哈希
 func HashPassword(plain string) (string, error) {
 	b, err := bcrypt.GenerateFromPassword([]byte(plain), bcrypt.DefaultCost)