refactor(diary): Service 层改用 DiaryEvent 枚举替代字符串事件

- journal_service: 3 处 (JournalCreated/Updated/Deleted)
- class_service: 2 处 (ClassCreated/StudentJoinedClass)
- comment_service: 1 处 (CommentCreated)
- topic_service: 1 处 (TopicAssigned)
- parent_service: 1 处 confirm_binding → ParentBound

保留 DomainEvent::new 的场景：
- class_service deactivate_class (diary.class.deactivated)
- parent_service bind_child (diary.parent.binding_requested)
- parent_service delete_child_data (diary.parent.data_deleted)
以上事件不在 DiaryEvent 枚举中（非核心创建事件）

测试: 509/509 全部通过

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

@@ -12,6 +12,8 @@ use crate::entity::{class_member, school_class};
 use crate::error::{DiaryError, DiaryResult};
 use erp_core::events::{DomainEvent, EventBus};
 
+use crate::event::DiaryEvent;
+
 /// 班级服务 — 6 位码生成、过期控制、成员管理
 pub struct ClassService;
 
@@ -84,14 +86,11 @@ impl ClassService {
         // 发布 ClassCreated 事件
         event_bus
             .publish(
-                DomainEvent::new(
-                    "diary.class.created",
-                    tenant_id,
-                    serde_json::json!({
-                        "class_id": id,
-                        "teacher_id": teacher_id,
-                    }),
-                ),
+                DiaryEvent::ClassCreated {
+                    class_id: id,
+                    teacher_id,
+                }
+                .to_domain_event(tenant_id),
                 db,
             )
             .await;
@@ -229,14 +228,11 @@ impl ClassService {
         // 8. 发布 StudentJoinedClass 事件
         event_bus
             .publish(
-                DomainEvent::new(
-                    "diary.class.student_joined",
-                    tenant_id,
-                    serde_json::json!({
-                        "class_id": class_id,
-                        "student_id": user_id,
-                    }),
-                ),
+                DiaryEvent::StudentJoinedClass {
+                    class_id,
+                    student_id: user_id,
+                }
+                .to_domain_event(tenant_id),
                 db,
             )
             .await;