GET /feed/:id

{
  id: string,
  owner: ResponseUserProfileSchema,
  image: ResponseImageSchema,      // 所有 feed 都包含（即使 mediaType=video/audio，这里是 cover/thumb）
  video?: ResponseFeedVideoSchema, // mediaType = "video" 时
  audio?: ResponseFeedAudioSchema, // mediaType = "audio" 时
  mediaType: "image" | "video" | "audio",
  originImageId: string,
  createdAt: string,
  likedNumber: number,
  flag: string[]                   // "nsfw" / "duplicate" / "ban"
}