feat: 评论/帖子编辑历史与编辑标记，内容锁定

- 新增 smartDiff 差异计算与 EditHistoryModal 编辑历史弹窗，替换原 CommentEditHistoryModal
- 新增 PostEditedMark 编辑标记展示
- 新增内容锁定机制（content_lock）防止并发编辑冲突
- 审核与通知服务适配

backend/service/admin_content.go

@@ -81,6 +81,7 @@ type AdminContentComment struct {
 	BoardID   uint       `json:"board_id"`
 	Content   string     `json:"content"`
 	Status    string     `json:"status"`
+	Edited    bool       `json:"edited"` // 相对创建已编辑（同评论侧规则），用于隐藏无修订的历史入口
 	Deleted   bool       `json:"deleted"`
 	DeletedAt *time.Time `json:"deleted_at,omitempty"`
 	CreatedAt time.Time  `json:"created_at"`
@@ -223,13 +224,14 @@ func (s *ModerationService) ListAdminComments(actor *Actor, status, keyword stri
 		Content   string
 		Status    string
 		CreatedAt time.Time
+		UpdatedAt time.Time
 		DeletedAt gorm.DeletedAt
 		BoardID   uint
 		PostTitle string
 		UserID    uint
 	}
 	var rows []row
-	if err := q.Select("comments.id, comments.post_id, comments.content, comments.status, comments.created_at, comments.deleted_at, posts.board_id, posts.title AS post_title, comments.user_id").
+	if err := q.Select("comments.id, comments.post_id, comments.content, comments.status, comments.created_at, comments.updated_at, comments.deleted_at, posts.board_id, posts.title AS post_title, comments.user_id").
 		Order("comments.id DESC").
 		Offset((page - 1) * adminContentPageSize).Limit(adminContentPageSize).
 		Scan(&rows).Error; err != nil {
@@ -276,6 +278,7 @@ func (s *ModerationService) ListAdminComments(actor *Actor, status, keyword stri
 			BoardID:   r.BoardID,
 			Content:   r.Content,
 			Status:    r.Status,
+			Edited:    r.UpdatedAt.Sub(r.CreatedAt) > time.Minute,
 			CreatedAt: r.CreatedAt,
 			Board:     boards[r.BoardID],
 			User:      users[r.UserID],
@@ -354,7 +357,8 @@ func (s *ModerationService) SoftDeletePost(actor *Actor, id uint, deleteType, de
 		"delete_reason": deleteReason,
 		"deleted_by":    actor.ID,
 	}
-	if err := s.db.Model(post).Updates(updates).Error; err != nil {
+	// UpdateColumns：删除元数据非内容编辑，不刷新 updated_at（详情页「更新于」由该字段推导）
+	if err := s.db.Model(post).UpdateColumns(updates).Error; err != nil {
 		return 0, err
 	}
 	if err := s.db.Delete(post).Error; err != nil {
@@ -375,7 +379,8 @@ func (s *ModerationService) RestorePost(actor *Actor, id uint) error {
 	if !post.DeletedAt.Valid {
 		return errors.New("帖子未被删除")
 	}
-	return s.db.Unscoped().Model(&model.Post{}).Where("id = ?", id).Update("deleted_at", nil).Error
+	// UpdateColumn：恢复非内容编辑，不刷新 updated_at
+	return s.db.Unscoped().Model(&model.Post{}).Where("id = ?", id).UpdateColumn("deleted_at", nil).Error
 }
 
 // PurgePost 彻底删除已软删帖子及其评论（仅站长）
@@ -391,6 +396,9 @@ func (s *ModerationService) PurgePost(actor *Actor, id uint) error {
 		return ErrContentNotDeleted
 	}
 	return s.db.Transaction(func(tx *gorm.DB) error {
+		if err := tx.Unscoped().Where("post_id = ?", id).Delete(&model.PostEditHistory{}).Error; err != nil {
+			return err
+		}
 		if err := tx.Unscoped().Where("post_id = ?", id).Delete(&model.Comment{}).Error; err != nil {
 			return err
 		}
@@ -430,7 +438,8 @@ func (s *ModerationService) SoftDeleteComment(actor *Actor, id uint, deleteType,
 		"delete_reason": deleteReason,
 		"deleted_by":    actor.ID,
 	}
-	if err := s.db.Model(cm).Updates(updates).Error; err != nil {
+	// UpdateColumns：删除元数据非内容编辑，不刷新 updated_at（该字段用于 edited 判定）
+	if err := s.db.Model(cm).UpdateColumns(updates).Error; err != nil {
 		return err
 	}
 	if err := s.db.Delete(cm).Error; err != nil {
@@ -461,7 +470,8 @@ func (s *ModerationService) RestoreComment(actor *Actor, id uint) error {
 		"delete_reason": "",
 		"deleted_by":    0,
 	}
-	if err := s.db.Unscoped().Model(&model.Comment{}).Where("id = ?", id).Updates(updates).Error; err != nil {
+	// UpdateColumns：恢复非内容编辑，不刷新 updated_at（该字段用于 edited 判定）
+	if err := s.db.Unscoped().Model(&model.Comment{}).Where("id = ?", id).UpdateColumns(updates).Error; err != nil {
 		return err
 	}
 	if cm.Status == model.ContentStatusPublished {