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/topic_assignment.rs

@@ -0,0 +1,52 @@
+// 主题布置 — 老师发布日记主题，学生提交对应日记
+
+use sea_orm::entity::prelude::*;
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Debug, PartialEq, DeriveEntityModel, Serialize, Deserialize)]
+#[sea_orm(table_name = "topic_assignments")]
+pub struct Model {
+    #[sea_orm(primary_key, auto_increment = false)]
+    pub id: Uuid,
+    pub tenant_id: Uuid,
+    /// 所属班级
+    pub class_id: Uuid,
+    /// 布置主题的老师
+    pub teacher_id: Uuid,
+    /// 主题标题
+    pub title: String,
+    /// 主题描述/要求
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub description: Option<String>,
+    /// 截止日期
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub due_date: Option<chrono::NaiveDate>,
+    /// 是否激活
+    pub is_active: bool,
+    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(
+        belongs_to = "super::school_class::Entity",
+        from = "Column::ClassId",
+        to = "super::school_class::Column::Id",
+        on_delete = "Cascade"
+    )]
+    SchoolClass,
+}
+
+impl Related<super::school_class::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::SchoolClass.def()
+    }
+}
+
+impl ActiveModelBehavior for ActiveModel {}