feat(comments): 软删占位、理由与恢复，管理删仅对访客打码

自删/管理删差异文案与理由可见性，评论者与管理员免马赛克，并支持恢复与处理人链接。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/router/router.go

@@ -151,6 +151,7 @@ func Setup(cfg *config.Config) (*gin.Engine, error) {
 		api.POST("/posts/:id/like", h.ToggleLike)
 		api.POST("/posts/:id/comments", middleware.RateLimitMiddleware(limiter, service.RateComment), h.CreateComment)
 		api.DELETE("/posts/:id/comments/:cid", h.DeleteComment)
+		api.PUT("/posts/:id/comments/:cid/restore", h.RestoreComment)
 		api.DELETE("/posts/:id/comments/:cid/purge", h.PurgeComment)
 		// 通知
 		api.GET("/notifications", h.Notifications)