Add user management for admin (CRUD + role + reset password)

Adds /api/users endpoints (admin only) plus /api/users/assignable
(admin + technician) used by the aftersales reassign picker. Guards
prevent self-demotion, self-deletion, and removing the last admin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

models/models.go

@@ -74,6 +74,27 @@ type UpdateProfileDTO struct {
 	Email string `json:"email" validate:"required,email"`
 }
 
+// CreateUserDTO 管理员创建用户请求
+type CreateUserDTO struct {
+	Username string `json:"username" validate:"required,min=3,max=50"`
+	Password string `json:"password" validate:"required,min=6"`
+	Name     string `json:"name" validate:"required"`
+	Email    string `json:"email" validate:"omitempty,email"`
+	Role     string `json:"role" validate:"required,oneof=admin technician user"`
+}
+
+// UpdateUserDTO 管理员更新用户信息请求
+type UpdateUserDTO struct {
+	Name  string `json:"name,omitempty"`
+	Email string `json:"email,omitempty" validate:"omitempty,email"`
+	Role  string `json:"role,omitempty" validate:"omitempty,oneof=admin technician user"`
+}
+
+// AdminResetPasswordDTO 管理员重置用户密码
+type AdminResetPasswordDTO struct {
+	NewPassword string `json:"newPassword" validate:"required,min=6"`
+}
+
 // GenerateSerialDTO 生成序列号请求数据
 type GenerateSerialDTO struct {
 	CompanyName string `json:"companyName" validate:"required"`