feat: 新增文章阅读量计数（已登录去重 + 访客防刷）

- Post 模型新增 ViewsCount 冗余计数字段
- 新建 PostReadLog 表（user_id + post_id 唯一索引），已登录用户每篇文章只计一次
- 新建 PostGuestReadLog 表（visitor_id + post_id 唯一索引），访客基于 Cookie 标识去重
- Repository 层新增 RecordRead / RecordGuestRead / IncrementViewsCount
- Service 层编排去重-计数逻辑，计数失败不影响页面渲染
- Controller 层 ShowPage + ShowAPI 触发计数，仅 approved 状态生效
- 双层防刷：唯一索引去重 + 2 秒冷却间隔
- 访客 Cookie (visitor_id) 基于 crypto/rand 生成，30 天有效期

cmd/server/main.go

@@ -27,7 +27,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("连接数据库失败: %v", err)
 	}
-	if err := db.AutoMigrate(&model.User{}, &model.AuditSubmission{}, &model.SiteSetting{}, &model.Notification{}, &model.Post{}, &model.UserCheckIn{}, &model.UserTask{}, &model.EnergyLog{}, &model.PostEnergizeLog{}, &model.DailyExpSummary{}, &model.Comment{}, &model.CommentMention{}, &model.CommunityFund{}, &model.FundLog{}, &model.PostLike{}, &model.PostDislike{}, &model.UserFollow{}, &model.DailyLikeSummary{}, &model.Folder{}, &model.FolderItem{}); err != nil {
+	if err := db.AutoMigrate(&model.User{}, &model.AuditSubmission{}, &model.SiteSetting{}, &model.Notification{}, &model.Post{}, &model.PostReadLog{}, &model.PostGuestReadLog{}, &model.UserCheckIn{}, &model.UserTask{}, &model.EnergyLog{}, &model.PostEnergizeLog{}, &model.DailyExpSummary{}, &model.Comment{}, &model.CommentMention{}, &model.CommunityFund{}, &model.FundLog{}, &model.PostLike{}, &model.PostDislike{}, &model.UserFollow{}, &model.DailyLikeSummary{}, &model.Folder{}, &model.FolderItem{}); err != nil {
 		log.Fatalf("数据库迁移失败: %v", err)
 	}