feat(user): require email when admin creates a user

- Add `email` as a required field in `createUserRequest`
- Enforce email format verification and database uniqueness checks in the admin user creation handler
- Update the admin user creation frontend modal with validation and form field
- Update the corresponding backend unit tests and regenerate Swagger docs

frontend/lib/services/admin/types.ts

@@ -340,6 +340,8 @@ export interface CreateUserRequest {
   password: string;
   /** 昵称 */
   nickname?: string;
+  /** 邮箱 */
+  email: string;
   /** 是否激活 */
   is_active?: boolean;
   /** 是否管理员 */