improvement(sns): 精简朋友圈媒体链路并移除手动挑图入口

- 移除 Server-Timing/X-SNS 阶段透出与前端媒体来源标记
- 移除本地候选图与手动匹配接口及预览交互
- /api/sns/media 默认仅保留远程下载解密路径，未命中直接返回 404
- 将导出所需的本地缓存匹配逻辑下沉到 sns_export_service

Author: 2977094657

frontend/composables/useApi.js

@@ -357,30 +357,6 @@ export const useApi = () => {
     return await request(url)
   }
 
-  // 朋友圈图片本地缓存候选（用于错图时手动选择）
-  const listSnsMediaCandidates = async (params = {}) => {
-    const query = new URLSearchParams()
-    if (params && params.account) query.set('account', params.account)
-    if (params && params.create_time != null) query.set('create_time', String(params.create_time))
-    if (params && params.width != null) query.set('width', String(params.width))
-    if (params && params.height != null) query.set('height', String(params.height))
-    if (params && params.limit != null) query.set('limit', String(params.limit))
-    if (params && params.offset != null) query.set('offset', String(params.offset))
-    const url = '/sns/media_candidates' + (query.toString() ? `?${query.toString()}` : '')
-    return await request(url)
-  }
-
-  // 保存朋友圈图片手动匹配结果（本机）
-  const saveSnsMediaPicks = async (data = {}) => {
-    return await request('/sns/media_picks', {
-      method: 'POST',
-      body: {
-        account: data.account || null,
-        picks: (data && data.picks && typeof data.picks === 'object' && !Array.isArray(data.picks)) ? data.picks : {}
-      }
-    })
-  }
-
   const openChatMediaFolder = async (params = {}) => {
     const query = new URLSearchParams()
     if (params && params.account) query.set('account', params.account)
@@ -667,8 +643,6 @@ export const useApi = () => {
     resolveAppMsg,
     listSnsTimeline,
     listSnsUsers,
-    listSnsMediaCandidates,
-    saveSnsMediaPicks,
     openChatMediaFolder,
     downloadChatEmoji,
     saveMediaKeys,