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/handler/moderation.go

@@ -193,41 +193,6 @@ func (h *Handlers) AdminPurgeComment(c *gin.Context) {
 	h.execContentAction(c, h.Moderation.PurgeComment)
 }
 
-// AdminCommentHistory 评论编辑历史（板域 scope；分页）
-func (h *Handlers) AdminCommentHistory(c *gin.Context) {
-	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
-	if err != nil || id == 0 {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的 ID"})
-		return
-	}
-	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
-	size, _ := strconv.Atoi(c.DefaultQuery("size", "10"))
-	items, total, err := h.Comment.ListEditHistory(middleware.CurrentActor(c), uint(id), page, size)
-	if err != nil {
-		switch {
-		case errors.Is(err, service.ErrCommentNotFound):
-			c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
-		case errors.Is(err, service.ErrCommentForbidden):
-			c.JSON(http.StatusForbidden, gin.H{"error": err.Error()})
-		default:
-			c.JSON(http.StatusInternalServerError, gin.H{"error": "获取编辑历史失败"})
-		}
-		return
-	}
-	if page < 1 {
-		page = 1
-	}
-	if size < 1 || size > 50 {
-		size = 10
-	}
-	c.JSON(http.StatusOK, gin.H{
-		"items": items,
-		"total": total,
-		"page":  page,
-		"size":  size,
-	})
-}
-
 func (h *Handlers) execContentAction(c *gin.Context, fn func(*service.Actor, uint) error) {
 	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
 	if err != nil || id == 0 {