fix(diary): 为所有 DTO 添加 Validate derive + handler 调用 validate()
DTO 验证规则:
- CreateJournalReq: title 1-200, tags ≤20
- UpdateJournalReq: title 1-200, tags ≤20
- CreateClassReq: name 1-50, school_name ≤100
- JoinClassReq: class_code = 6位
- UpdateClassReq: name 1-50, school_name ≤100
- SyncReq: changes ≤100 条
- CreateTopicReq: title 1-200, description ≤2000
- UpdateTopicReq: title 1-200, description ≤2000
- CreateCommentReq: content 1-1000
- CreateStickerPackReq: name 1-50, description ≤500
- UpdateStickerPackReq: name 1-50, description ≤500
- CreateStickerReq: name 1-30, image_url 1-500
- BindChildReq/DeleteChildDataReq: Validate derive (Uuid 已由 serde 验证)
Handler 调用: validate() 放在 require_permission() 之前（先验证输入再检查权限）
审计 ID: 5a-C01, 5a-C02, 5a-C03
b81a972245