feat: 实现评论系统（后端 + 前端）

- 新增 Comment/CommentMention 模型，Post 新增 CommentsCount 冗余计数器
- 新增 CommentRepo（CRUD + @搜索 LV3+ + 回复统计 + 文章作者查询）
- 新增 CommentService（发表/回复/删除/@解析/通知 NotifyComment/NotifyCommentReply）
- 新增 CommentController（6 个 API 端点），commentUseCase ISP 接口
- 新增评论路由（公开读取 + 需登录写入），依赖注入，错误哨兵，数据库迁移
- 前端评论区 HTML/CSS/JS：分页加载、回复折叠展开、内联回复表单、@提及自动补全

internal/model/notification.go

@@ -2,11 +2,13 @@ package model
 
 // 消息/通知类型常量
 const (
-	NotifyAuditApproved = "audit_approved"
-	NotifyAuditRejected = "audit_rejected"
-	NotifyPostApproved  = "post_approved"
-	NotifyPostRejected  = "post_rejected"
-	NotifyLevelUp       = "level_up"
+	NotifyAuditApproved  = "audit_approved"
+	NotifyAuditRejected  = "audit_rejected"
+	NotifyPostApproved   = "post_approved"
+	NotifyPostRejected   = "post_rejected"
+	NotifyLevelUp        = "level_up"
+	NotifyComment        = "comment"
+	NotifyCommentReply   = "comment_reply"
 )
 
 // Notification 消息/通知模型
@@ -28,6 +30,8 @@ var NotifyTypeNames = map[string]string{
 	NotifyPostApproved:  "稿件审核",
 	NotifyPostRejected:  "稿件审核",
 	NotifyLevelUp:       "等级提升",
+	NotifyComment:       "评论",
+	NotifyCommentReply:  "回复",
 }
 
 // NotificationListResult 消息列表查询结果