feat(moderation): 帖评删除占位感知与硬删清场

管理删帖须填类型与理由并展示专用页；评论软删占位、版主可硬删不占楼。

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.DELETE("/posts/:id/comments/:cid/purge", h.PurgeComment)
 		// 通知
 		api.GET("/notifications", h.Notifications)
 		api.GET("/notifications/unread-count", h.UnreadCount)