feat(comments): 支持编辑与管理可见历史，采纳展示改用楼层号

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/model/models.go

@@ -321,6 +321,15 @@ type Comment struct {
 	User User `gorm:"foreignKey:UserID" json:"user,omitempty"`
 }
 
+// CommentEditHistory 评论修订快照（每次编辑前的旧正文；仅管理可见）
+type CommentEditHistory struct {
+	ID         uint      `gorm:"primaryKey" json:"id"`
+	CommentID  uint      `gorm:"index;not null" json:"comment_id"`
+	EditorID   uint      `gorm:"index;not null" json:"editor_id"`
+	OldContent string    `gorm:"type:text;not null" json:"old_content"`
+	CreatedAt  time.Time `gorm:"index" json:"created_at"`
+}
+
 // Like 点赞记录（联合唯一索引防止重复点赞）
 type Like struct {
 	ID        uint      `gorm:"primaryKey" json:"id"`