feat(media): 重构媒体下载，支持 xhs/dy/ks/bili/wb 五平台

旧实现只覆盖 4 个平台，且把整个文件读进内存、无重试与完整性校验，
代码按平台复制粘贴了 4 份。本次用统一下载器替换：

- 新增 media_downloader/：流式写入、Range 续传、指数退避重试、大小校验、
  路径穿越防护；B 站 DASH 音视频分轨下载后交由 ffmpeg 无损合流
- 新增 media_platform/<平台>/media.py：从平台原始响应提取媒体地址，
  与下载器解耦；快手首次接入下载能力
- 开关：config.ENABLE_GET_MEDIA 与 --get_media，并打通 API/WebUI；
  同时修正旧配置项 ENABLE_GET_MEIDAS 的拼写
- 落盘按帖子聚合：{SAVE_DATA_PATH 或 data}/{platform}/media/{内容ID}/
- B 站装好 ffmpeg 时走 DASH 最高画质，否则降级 mp4 直链（产物 video-durl.mp4，
  避免低清文件阻塞后续的高清路径）
- 删除 4 个 *_store_media.py、AbstractStoreImage/Video 及各 client 的媒体 GET 方法

媒体下载失败只记录日志，不中断爬取主流程。

webui/src/components/config/CrawlerConfigPanel.tsx

@@ -1,7 +1,7 @@
 import type { ComponentType, ReactNode, KeyboardEvent } from 'react'
 import { useState } from 'react'
 import { useTranslation } from 'react-i18next'
-import { Database, Globe, KeyRound, MessageSquare, Play, Square, X } from 'lucide-react'
+import { Database, Globe, Image as ImageIcon, KeyRound, MessageSquare, Play, Square, X } from 'lucide-react'
 import { Label } from '@/components/ui/label'
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from '@/components/ui/select'
 import { Input } from '@/components/ui/input'
@@ -371,6 +371,23 @@ export function CrawlerConfigPanel() {
               <p className="text-xs font-mono text-cyber-text-primary">{t('field.subComments')}</p>
             </div>
 
+            <div className="flex items-center gap-3 rounded-lg border border-cyber-border-subtle bg-cyber-bg-tertiary/30 p-2.5 hover:border-cyber-border-DEFAULT transition-colors">
+              <Checkbox
+                checked={config.enable_media}
+                onCheckedChange={(checked) => updateConfig({ enable_media: checked === true })}
+                disabled={isDisabled}
+              />
+              <div className="min-w-0 flex-1">
+                <div className="flex items-center gap-2">
+                  <ImageIcon className="h-3.5 w-3.5 text-cyber-text-secondary" />
+                  <p className="text-xs font-mono text-cyber-text-primary">{t('field.downloadMedia')}</p>
+                </div>
+                <p className="text-[10px] text-cyber-text-muted leading-snug">
+                  {t('field.downloadMediaHint')}
+                </p>
+              </div>
+            </div>
+
             <div className="flex items-center gap-3 rounded-lg border border-cyber-border-subtle bg-cyber-bg-tertiary/30 p-2.5 hover:border-cyber-border-DEFAULT transition-colors">
               <Checkbox
                 checked={config.headless}

webui/src/i18n/locales/en-US/config.json

@@ -55,6 +55,8 @@
     "saveFormatPlaceholder": "Select format",
     "commentExtraction": "Comment Extraction",
     "subComments": "Sub-comments",
+    "downloadMedia": "DOWNLOAD_MEDIA",
+    "downloadMediaHint": "Download cover and video (images for image posts). Supports xhs/dy/ks/bili/wb",
     "headlessMode": "HEADLESS_MODE",
     "headlessModeHint": "Run browser without GUI"
   },

webui/src/i18n/locales/zh-CN/config.json

@@ -55,6 +55,8 @@
     "saveFormatPlaceholder": "选择格式",
     "commentExtraction": "评论抓取",
     "subComments": "子评论",
+    "downloadMedia": "下载媒体",
+    "downloadMediaHint": "下载封面与视频（图文笔记下载图片），支持小红书/抖音/快手/B站/微博",
     "headlessMode": "无头模式",
     "headlessModeHint": "无 GUI 运行浏览器"
   },

webui/src/lib/api.ts

@@ -17,6 +17,7 @@ export interface CrawlerConfig {
   start_page: number
   enable_comments: boolean
   enable_sub_comments: boolean
+  enable_media: boolean
   save_option: string
   cookies: string
   headless: boolean

webui/src/store/crawlerStore.ts

@@ -56,6 +56,7 @@ const defaultConfig: CrawlerConfig = {
   start_page: 1,
   enable_comments: true,
   enable_sub_comments: false,
+  enable_media: false,
   save_option: 'json',
   cookies: '',
   headless: false,

webui/src/types/crawler.ts

@@ -8,6 +8,7 @@ export interface CrawlerConfig {
   start_page: number
   enable_comments: boolean
   enable_sub_comments: boolean
+  enable_media: boolean  // 是否下载媒体（封面/视频/图文图片），支持 xhs/dy/ks/bili/wb
   save_option: string
   cookies: string
   headless: boolean