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/service/repository.go

@@ -115,3 +115,28 @@ type energyStore = EnergyStore
 type energyNotifier interface {
 	Create(userID uint, notifyType, title, content string, relatedID *uint) error
 }
+
+// commentStore CommentService 所需的最小仓储接口（ISP）
+type commentStore interface {
+	Create(comment *model.Comment) error
+	UpdateRootID(id, rootID uint) error
+	FindByID(id uint) (*model.Comment, error)
+	FindRootComments(postID uint, offset, limit int) ([]model.Comment, int64, error)
+	FindReplies(rootID uint) ([]model.Comment, error)
+	SoftDelete(id uint) error
+	IncrCommentsCount(postID uint) error
+	DecrCommentsCount(postID uint) error
+	CreateMention(mention *model.CommentMention) error
+	FindPostIDByComment(commentID uint) (uint, error)
+	FindPostAuthorID(postID uint) (uint, error)
+	SearchUsersByLevel(keyword string, minLevel, limit int) ([]struct {
+		UID      string
+		Username string
+	}, error)
+	GetUserUIDByID(userID uint) (string, error)
+}
+
+// commentNotifier CommentService 所需的通知接口（ISP）
+type commentNotifier interface {
+	Create(userID uint, notifyType, title, content string, relatedID *uint) error
+}