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/post.go

@@ -89,6 +89,7 @@ func (h *Handlers) PostDetail(c *gin.Context) {
 	if claims != nil {
 		post.Liked = h.Like.HasLiked(post.ID, claims.ID)
 	}
+	h.attachNecroReplyHint(claims, post)
 	c.JSON(http.StatusOK, gin.H{"post": post})
 }
 
@@ -96,7 +97,7 @@ func (h *Handlers) PostDetail(c *gin.Context) {
 type CreatePostRequest struct {
 	BoardID       uint   `json:"board_id" binding:"required"`
 	Title         string `json:"title" binding:"required,min=1,max=256"`
-	Content       string `json:"content" binding:"required,min=1"`
+	Content       string `json:"content" binding:"required,min=1,max=20000"`
 	Tags          string `json:"tags"`
 	PostType      string `json:"post_type"`
 	TypeMeta      string `json:"type_meta"`
@@ -138,7 +139,7 @@ func (h *Handlers) CreatePost(c *gin.Context) {
 // UpdatePostRequest 更新帖子请求
 type UpdatePostRequest struct {
 	Title         string  `json:"title" binding:"omitempty,min=1,max=256"`
-	Content       string  `json:"content" binding:"omitempty,min=1"`
+	Content       string  `json:"content" binding:"omitempty,min=1,max=20000"`
 	Tags          string  `json:"tags"`
 	TypeMeta      *string `json:"type_meta"`
 	AttachmentIDs *[]uint `json:"attachment_ids"`
@@ -172,6 +173,42 @@ func (h *Handlers) UpdatePost(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"post": post})
 }
 
+// PostEditHistory 帖子编辑历史（登录即可见；分页）
+func (h *Handlers) PostEditHistory(c *gin.Context) {
+	claims := middleware.CurrentUser(c)
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil || id == 0 {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的帖子 ID"})
+		return
+	}
+	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
+	size, _ := strconv.Atoi(c.DefaultQuery("size", "10"))
+	items, total, err := h.Post.ListEditHistory(h.loadActor(claims.ID), uint(id), page, size)
+	if err != nil {
+		switch {
+		case errors.Is(err, service.ErrPostNotFound):
+			c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
+		case errors.Is(err, service.ErrPostForbidden):
+			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,
+	})
+}
+
 // UnlockPostContent 积分解锁正文
 func (h *Handlers) UnlockPostContent(c *gin.Context) {
 	claims := middleware.CurrentUser(c)
@@ -239,6 +276,7 @@ func (h *Handlers) UnlockPostPassword(c *gin.Context) {
 	if claims != nil {
 		post.Liked = h.Like.HasLiked(post.ID, claims.ID)
 	}
+	h.attachNecroReplyHint(claims, post)
 	c.JSON(http.StatusOK, gin.H{"post": post})
 }
 
@@ -539,6 +577,52 @@ func (h *Handlers) ToggleRecommend(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"recommended": rec})
 }
 
+// TogglePostLock 切换帖子手动锁定（管理员及以上；锁定后普通用户不可编辑/回复，staff 豁免）
+func (h *Handlers) TogglePostLock(c *gin.Context) {
+	if !h.requireAdminOrAbove(c) {
+		return
+	}
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的帖子 ID"})
+		return
+	}
+	locked, err := h.Post.ToggleLock(uint(id))
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"locked": locked})
+}
+
+// attachNecroReplyHint 旧帖回复确认提示：仅当 viewer 已登录、非 staff、规则开启、
+// 帖子判定为旧帖、且其评论将直发（待审不扣分不提醒，不弹窗）时返回
+func (h *Handlers) attachNecroReplyHint(claims *service.UserClaims, detail *service.PostDetail) {
+	if claims == nil || detail == nil || detail.Tombstone || detail.NecroReply != nil {
+		return
+	}
+	if model.IsStaff(model.Role(claims.Role)) {
+		return
+	}
+	afterHours, err1 := h.Setting.NecroReplyAfterHours()
+	penalty, err2 := h.Setting.NecroReplyPenalty()
+	if err1 != nil || err2 != nil || afterHours <= 0 || penalty <= 0 {
+		return
+	}
+	// 旧帖基准：最后回复时间；无回复（null）回落发帖时间
+	baseline := detail.CreatedAt
+	if detail.LastReplyAt != nil {
+		baseline = *detail.LastReplyAt
+	}
+	if !service.IsNecroReply(baseline, afterHours, time.Now()) {
+		return
+	}
+	if h.resolvePublishStatus(claims.ID, claims.Role) != model.ContentStatusPublished {
+		return
+	}
+	detail.NecroReply = &service.NecroReplyHint{AfterHours: afterHours, Penalty: penalty}
+}
+
 // requireAdminOrAbove 置顶/加精仅管理员及以上可用；以 DB Actor 为准
 func (h *Handlers) requireAdminOrAbove(c *gin.Context) bool {
 	claims := middleware.CurrentUser(c)
@@ -556,7 +640,9 @@ func respondPostModError(c *gin.Context, err error) {
 	case errors.Is(err, service.ErrPostNotFound):
 		c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
 	case errors.Is(err, service.ErrPostForbidden),
-		errors.Is(err, service.ErrAuthorProtected):
+		errors.Is(err, service.ErrAuthorProtected),
+		errors.Is(err, service.ErrPostLocked),
+		errors.Is(err, service.ErrPostEditLocked):
 		c.JSON(http.StatusForbidden, gin.H{"error": err.Error()})
 	case errors.Is(err, service.ErrPollOptionsLocked),
 		errors.Is(err, service.ErrLotteryPrizesLocked):