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

app/lib/data/models/school_class.dart

@@ -0,0 +1,112 @@
+// 班级数据模型 — 手写不可变类（避免 build_runner 依赖）
+
+import 'package:uuid/uuid.dart';
+
+/// 班级 — 老师创建，学生通过班级码加入
+///
+/// 班级码安全规则：
+/// - 6 位字母数字混合（62^6 约 568 亿种组合）
+/// - 有效期控制（学期结束自动失效）
+/// - 连续 5 次错误锁定 30 分钟
+class SchoolClass {
+  final String id;
+  final String name;
+  final String schoolName;
+  final String teacherId;
+  final String classCode;
+  final int memberCount;
+  final bool isActive;
+  final DateTime? expiresAt;
+  final DateTime createdAt;
+  final DateTime updatedAt;
+
+  const SchoolClass({
+    required this.id,
+    required this.name,
+    required this.schoolName,
+    required this.teacherId,
+    required this.classCode,
+    this.memberCount = 0,
+    this.isActive = true,
+    this.expiresAt,
+    required this.createdAt,
+    required this.updatedAt,
+  });
+
+  SchoolClass copyWith({
+    String? id,
+    String? name,
+    String? schoolName,
+    String? teacherId,
+    String? classCode,
+    int? memberCount,
+    bool? isActive,
+    DateTime? expiresAt,
+    bool clearExpiresAt = false,
+    DateTime? createdAt,
+    DateTime? updatedAt,
+  }) =>
+      SchoolClass(
+        id: id ?? this.id,
+        name: name ?? this.name,
+        schoolName: schoolName ?? this.schoolName,
+        teacherId: teacherId ?? this.teacherId,
+        classCode: classCode ?? this.classCode,
+        memberCount: memberCount ?? this.memberCount,
+        isActive: isActive ?? this.isActive,
+        expiresAt: clearExpiresAt ? null : (expiresAt ?? this.expiresAt),
+        createdAt: createdAt ?? this.createdAt,
+        updatedAt: updatedAt ?? this.updatedAt,
+      );
+
+  Map<String, dynamic> toJson() => {
+        'id': id,
+        'name': name,
+        'school_name': schoolName,
+        'teacher_id': teacherId,
+        'class_code': classCode,
+        'member_count': memberCount,
+        'is_active': isActive,
+        'expires_at': expiresAt?.toIso8601String(),
+        'created_at': createdAt.toIso8601String(),
+        'updated_at': updatedAt.toIso8601String(),
+      };
+
+  factory SchoolClass.fromJson(Map<String, dynamic> json) => SchoolClass(
+        id: json['id'] as String,
+        name: json['name'] as String,
+        schoolName: json['school_name'] as String,
+        teacherId: json['teacher_id'] as String,
+        classCode: json['class_code'] as String,
+        memberCount: (json['member_count'] as int?) ?? 0,
+        isActive: (json['is_active'] as bool?) ?? true,
+        expiresAt: json['expires_at'] != null
+            ? DateTime.parse(json['expires_at'] as String)
+            : null,
+        createdAt: DateTime.parse(json['created_at'] as String),
+        updatedAt: DateTime.parse(json['updated_at'] as String),
+      );
+
+  /// 创建新班级的工厂方法 — 自动生成 id 和时间戳
+  factory SchoolClass.create({
+    required String name,
+    required String schoolName,
+    required String teacherId,
+    required String classCode,
+    DateTime? expiresAt,
+  }) {
+    final now = DateTime.now();
+    return SchoolClass(
+      id: const Uuid().v4(),
+      name: name,
+      schoolName: schoolName,
+      teacherId: teacherId,
+      classCode: classCode,
+      memberCount: 0,
+      isActive: true,
+      expiresAt: expiresAt,
+      createdAt: now,
+      updatedAt: now,
+    );
+  }
+}