feat(diary): 添加 15 个 SeaORM 实体和数据库迁移 (Phase B1)

实体:
- journal_entry: 日记核心表 (心情/天气/标签/版本)
- journal_element: 日记元素 (文字/图片/贴纸/手写/胶带)
- handwriting_stroke: 手写笔画 (独立大字段表)
- school_class: 班级 (6位码/过期控制)
- class_member: 班级成员 (复合PK)
- topic_assignment: 主题布置
- comment: 老师点评
- sticker_pack + sticker: 贴纸包和贴纸
- template: 日记模板
- achievement + user_achievement: 成就系统
- parent_child_binding: 家长-孩子绑定 (PIPL)
- teacher_profile: 老师档案
- user_settings: 用户设置

迁移 (000170-000184):
- 15 个建表迁移 + 索引 + RLS 策略 + 种子数据
- 所有表含 tenant_id 多租户隔离
- 软删除 + 乐观锁版本号
- 外键级联删除
- 暖记权限注册到基座 permissions 表

验证: cargo check 通过, 425 个测试全通过

crates/erp-diary/src/entity/school_class.rs

@@ -0,0 +1,65 @@
+// 班级 — 老师创建，学生通过班级码加入
+
+use sea_orm::entity::prelude::*;
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Debug, PartialEq, DeriveEntityModel, Serialize, Deserialize)]
+#[sea_orm(table_name = "school_classes")]
+pub struct Model {
+    #[sea_orm(primary_key, auto_increment = false)]
+    pub id: Uuid,
+    pub tenant_id: Uuid,
+    /// 班级名称
+    pub name: String,
+    /// 学校名称
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub school_name: Option<String>,
+    /// 创建者（老师）ID
+    pub teacher_id: Uuid,
+    /// 6 位班级码（字母数字混合，62^6 ≈ 568 亿种组合）
+    pub class_code: String,
+    /// 成员数量（缓存字段）
+    pub member_count: i32,
+    /// 是否激活
+    pub is_active: bool,
+    /// 班级码过期时间（学期结束自动失效）
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub expires_at: Option<DateTimeUtc>,
+    pub created_at: DateTimeUtc,
+    pub updated_at: DateTimeUtc,
+    pub created_by: Uuid,
+    pub updated_by: Uuid,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub deleted_at: Option<DateTimeUtc>,
+    pub version: i32,
+}
+
+#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
+pub enum Relation {
+    #[sea_orm(has_many = "super::class_member::Entity")]
+    ClassMember,
+    #[sea_orm(has_many = "super::topic_assignment::Entity")]
+    TopicAssignment,
+    #[sea_orm(has_many = "super::journal_entry::Entity")]
+    JournalEntry,
+}
+
+impl Related<super::class_member::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::ClassMember.def()
+    }
+}
+
+impl Related<super::topic_assignment::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::TopicAssignment.def()
+    }
+}
+
+impl Related<super::journal_entry::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::JournalEntry.def()
+    }
+}
+
+impl ActiveModelBehavior for ActiveModel {}