feat(diary): 数据层 + 班级系统 (Phase F1 + B3)

Flutter 数据层 (Phase F1):
- journal_entry.dart: 日记数据模型 (Mood/Weather/tags/version)
- journal_element.dart: 元素模型 (text/image/sticker/handwriting_ref/tape)
- school_class.dart: 班级模型
- user_settings.dart: 用户设置 (主题/画笔/字号)
- isar_database.dart: Isar 初始化
- api_client.dart: Dio + JWT注入 + 离线感知 + 401处理
- journal_repository.dart: 抽象接口 + InMemory实现 (乐观锁)
- sync_engine.dart: WiFi同步 + 操作队列 + 重试(5次) + 快照持久化

Rust 班级系统 (Phase B3):
- class_service.rs: 创建班级(6位码) + 加入班级 + 成员管理
- topic_service.rs: 老师布置主题 + 主题列表
- comment_service.rs: 老师点评 + 评语列表
- class_handler.rs: 5个API端点 + 权限守卫
- topic_handler.rs: 2个API端点
- comment_handler.rs: 2个API端点
- dto.rs: 新增5个DTO (ClassMemberResp/CreateTopicReq/TopicResp/CreateCommentReq/CommentResp)
- 6条新路由注册

验证: cargo check 通过, 433测试全绿, flutter analyze 1 warning

crates/erp-diary/src/dto.rs

@@ -123,3 +123,58 @@ pub struct ConflictInfo {
     pub local_version: i32,
     pub server_version: i32,
 }
+
+// ========== 班级成员 ==========
+
+/// 班级成员响应
+#[derive(Debug, Serialize, ToSchema)]
+pub struct ClassMemberResp {
+    pub user_id: uuid::Uuid,
+    pub role: String,
+    pub nickname: Option<String>,
+    pub joined_at: chrono::DateTime<chrono::Utc>,
+}
+
+// ========== 主题布置 ==========
+
+/// 创建主题请求
+#[derive(Debug, Deserialize, ToSchema)]
+pub struct CreateTopicReq {
+    /// 主题标题
+    pub title: String,
+    /// 主题描述/要求
+    pub description: Option<String>,
+    /// 截止日期
+    pub due_date: Option<chrono::NaiveDate>,
+}
+
+/// 主题响应
+#[derive(Debug, Serialize, ToSchema)]
+pub struct TopicResp {
+    pub id: uuid::Uuid,
+    pub class_id: uuid::Uuid,
+    pub teacher_id: uuid::Uuid,
+    pub title: String,
+    pub description: Option<String>,
+    pub due_date: Option<chrono::NaiveDate>,
+    pub is_active: bool,
+}
+
+// ========== 评语 ==========
+
+/// 创建评语请求
+#[derive(Debug, Deserialize, ToSchema)]
+pub struct CreateCommentReq {
+    /// 评语内容
+    pub content: String,
+}
+
+/// 评语响应
+#[derive(Debug, Serialize, ToSchema)]
+pub struct CommentResp {
+    pub id: uuid::Uuid,
+    pub journal_id: uuid::Uuid,
+    pub author_id: uuid::Uuid,
+    pub content: String,
+    pub created_at: chrono::DateTime<chrono::Utc>,
+}