feat: 完成 opsx-licence 许可证管理平台全栈实现

基于 opsx-web-lite 框架（Go+Gin+Vue3+Element Plus）构建许可证管理服务器，
业务域参考 Licence 服务器 Web 版。

【后端 - 4层架构】
- 认证：单 Admin 表 + JWT（HS256）
- 核心授权域：Product / Customer / License / Activation
  · License 生成/验证/激活/机器绑定/心跳/续期/吊销
  · 客户端契约接口：/api/v1/license/{verify,activate,deactivate,heartbeat}
- 定价订单：Plan / Order / 支付网关配置
- 防重复购买：CustomerEntitlement + 幂等支付 webhook + 手动发放
- 分析审计：Dashboard 统计 / Analytics / AuditLog / Setting 管理

【前端】
- Dashboard（ECharts 图表）、产品、客户、License、计划、订单、权益、日志、设置
- 9 个完整 CRUD 业务页面 + 登录/布局

【部署】
- SQLite + 单二进制（//go:embed 嵌入前端）
- 开发：./run.sh start（前端5181/后端9001）
- 生产：./run.sh build && ./run.sh binary start
- 默认账号：admin/admin

Author: Logan.Li

backend/internal/service/customer/customer_service.go

@@ -0,0 +1,146 @@
+package customer
+
+import (
+	"backend/internal/dto"
+	"backend/internal/model"
+	"errors"
+
+	"gorm.io/gorm"
+)
+
+// CustomerService 客户业务逻辑层 // Logan.Li
+type CustomerService struct {
+	db *gorm.DB
+}
+
+// NewCustomerService 创建 service 实例 // Logan.Li
+func NewCustomerService(db *gorm.DB) *CustomerService {
+	return &CustomerService{db: db}
+}
+
+// List 分页查询客户 // Logan.Li
+func (s *CustomerService) List(page, pageSize int, keyword string) ([]dto.CustomerResponse, int64, error) {
+	var customers []model.Customer
+	var total int64
+
+	query := s.db.Model(&model.Customer{})
+	if keyword != "" {
+		query = query.Where("name LIKE ? OR email LIKE ? OR company LIKE ?", "%"+keyword+"%", "%"+keyword+"%", "%"+keyword+"%")
+	}
+
+	query.Count(&total)
+	if err := query.Order("created_at DESC").
+		Offset((page - 1) * pageSize).Limit(pageSize).
+		Find(&customers).Error; err != nil {
+		return nil, 0, err
+	}
+
+	results := make([]dto.CustomerResponse, 0, len(customers))
+	for _, c := range customers {
+		results = append(results, toCustomerResponse(c))
+	}
+	return results, total, nil
+}
+
+// GetAll 获取全部客户（下拉选项用） // Logan.Li
+func (s *CustomerService) GetAll() ([]dto.CustomerResponse, error) {
+	var customers []model.Customer
+	if err := s.db.Where("status = ?", 1).Order("created_at DESC").Find(&customers).Error; err != nil {
+		return nil, err
+	}
+	results := make([]dto.CustomerResponse, 0, len(customers))
+	for _, c := range customers {
+		results = append(results, toCustomerResponse(c))
+	}
+	return results, nil
+}
+
+// GetByID 根据 ID 获取客户 // Logan.Li
+func (s *CustomerService) GetByID(id uint) (*dto.CustomerResponse, error) {
+	var customer model.Customer
+	if err := s.db.First(&customer, id).Error; err != nil {
+		return nil, errors.New("客户不存在")
+	}
+	resp := toCustomerResponse(customer)
+	return &resp, nil
+}
+
+// Create 创建客户 // Logan.Li
+func (s *CustomerService) Create(req dto.CustomerCreate) (*dto.CustomerResponse, error) {
+	status := 1
+	if req.Status != nil {
+		status = *req.Status
+	}
+	customer := model.Customer{
+		Name:    req.Name,
+		Company: req.Company,
+		Email:   req.Email,
+		Phone:   req.Phone,
+		Address: req.Address,
+		Notes:   req.Notes,
+		Status:  status,
+	}
+	if err := s.db.Create(&customer).Error; err != nil {
+		return nil, errors.New("创建客户失败")
+	}
+	resp := toCustomerResponse(customer)
+	return &resp, nil
+}
+
+// Update 更新客户 // Logan.Li
+func (s *CustomerService) Update(id uint, req dto.CustomerUpdate) (*dto.CustomerResponse, error) {
+	var customer model.Customer
+	if err := s.db.First(&customer, id).Error; err != nil {
+		return nil, errors.New("客户不存在")
+	}
+
+	updates := map[string]interface{}{
+		"name":    req.Name,
+		"company": req.Company,
+		"email":   req.Email,
+		"phone":   req.Phone,
+		"address": req.Address,
+		"notes":   req.Notes,
+	}
+	if req.Status != nil {
+		updates["status"] = *req.Status
+	}
+
+	if err := s.db.Model(&customer).Updates(updates).Error; err != nil {
+		return nil, errors.New("更新客户失败")
+	}
+	s.db.First(&customer, id)
+	resp := toCustomerResponse(customer)
+	return &resp, nil
+}
+
+// Delete 删除客户（软删除） // Logan.Li
+func (s *CustomerService) Delete(id uint) error {
+	var count int64
+	s.db.Model(&model.License{}).Where("customer_id = ?", id).Count(&count)
+	if count > 0 {
+		return errors.New("该客户下存在 License，无法删除")
+	}
+	return s.db.Delete(&model.Customer{}, id).Error
+}
+
+// UpdateStatus 切换客户状态 // Logan.Li
+func (s *CustomerService) UpdateStatus(id uint, status int) error {
+	return s.db.Model(&model.Customer{}).Where("id = ?", id).Update("status", status).Error
+}
+
+// toCustomerResponse 模型转响应 DTO // Logan.Li
+func toCustomerResponse(c model.Customer) dto.CustomerResponse {
+	return dto.CustomerResponse{
+		ID:        c.ID,
+		Name:      c.Name,
+		Company:   c.Company,
+		Email:     c.Email,
+		Phone:     c.Phone,
+		Address:   c.Address,
+		Notes:     c.Notes,
+		Status:    c.Status,
+		CreatedAt: c.CreatedAt.Format("2006-01-02 15:04:05"),
+		UpdatedAt: c.UpdatedAt.Format("2006-01-02 15:04:05"),
+	}
+}