fix: 空评论列表 500，并完善撤回审计与角色说明

空切片 JSON 统一为数组；撤回保留正文供站长阅读，成员侧居中提示与重新编辑；头像支持拖放；角色弹窗补全职责说明并汇入封禁。

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

backend/model/models.go

@@ -318,5 +318,7 @@ type ChatMessage struct {
 	CreatedAt  time.Time      `gorm:"index:idx_chat_room_created,priority:2" json:"created_at"`
 	DeletedAt  gorm.DeletedAt `gorm:"index" json:"-"`
 
-	Sender User `gorm:"foreignKey:SenderID" json:"sender,omitempty"`
+	Sender   User  `gorm:"foreignKey:SenderID" json:"sender,omitempty"`
+	// RecalledBy=0 时不建 FK；Preload 仅用于已撤回消息展示撤回者昵称
+	Recaller *User `gorm:"foreignKey:RecalledBy;constraint:-" json:"recaller,omitempty"`
 }