fix(diary): 修复 class_service unwrap() — 改为从 Model 安全取值

- join_class 中 member_count.unwrap() 和 version.unwrap() 替换为
  在 Model → ActiveModel 转换前直接读取 model 字段值
- 消除潜在的 panic 风险，保持代码可读性

审计 ID: B-01

crates/erp-diary/src/service/class_service.rs

@@ -193,11 +193,12 @@ impl ClassService {
         member_model.insert(db).await?;
 
         // 6. 更新 member_count
+        let current_count = class_model.member_count;
+        let current_version = class_model.version;
         let mut active_class: school_class::ActiveModel = class_model.into();
-        let new_count = active_class.member_count.unwrap() + 1;
-        active_class.member_count = Set(new_count);
+        active_class.member_count = Set(current_count + 1);
         active_class.updated_at = Set(now);
-        active_class.version = Set(active_class.version.unwrap() + 1);
+        active_class.version = Set(current_version + 1);
         let updated_class = active_class.update(db).await?;
 
         // 7. 成功加入 → 清除错误计数