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。

media_platform/weibo/client.py

@@ -65,7 +65,6 @@ class WeiboClient(ProxyRefreshMixin):
         self.cookie_urls = [self._host]
         self.playwright_page = playwright_page
         self.cookie_dict = cookie_dict
-        self._image_agent_host = "https://i1.wp.com/"
         # Initialize proxy pool (from ProxyRefreshMixin)
         self.init_proxy_pool(proxy_ip_pool)
 
@@ -277,33 +276,6 @@ class WeiboClient(ProxyRefreshMixin):
                 utils.logger.info(f"[WeiboClient.get_note_info_by_id] $render_data value not found")
                 return dict()
 
-    async def get_note_image(self, image_url: str) -> bytes:
-        image_url = image_url[8:]  # Remove https://
-        sub_url = image_url.split("/")
-        image_url = ""
-        for i in range(len(sub_url)):
-            if i == 1:
-                image_url += "large/"  # Get high-resolution images
-            elif i == len(sub_url) - 1:
-                image_url += sub_url[i]
-            else:
-                image_url += sub_url[i] + "/"
-        # Weibo image hosting has anti-hotlinking, so proxy access is needed
-        # Since Weibo images are accessed through i1.wp.com, we need to concatenate the URL
-        final_uri = (f"{self._image_agent_host}"
-                     f"{image_url}")
-        async with make_async_client(proxy=self.proxy) as client:
-            try:
-                response = await client.request("GET", final_uri, timeout=self.timeout)
-                response.raise_for_status()
-                if not response.reason_phrase == "OK":
-                    utils.logger.error(f"[WeiboClient.get_note_image] request {final_uri} err, res:{response.text}")
-                    return None
-                else:
-                    return response.content
-            except httpx.HTTPError as exc:  # some wrong when call httpx.request method, such as connection error, client error, server error or response status code is not 2xx
-                utils.logger.error(f"[DouYinClient.get_aweme_media] {exc.__class__.__name__} for {exc.request.url} - {exc}")    # Keep original exception type name for developer debugging
-                return None
 
     async def get_creator_container_info(self, creator_id: str) -> Dict:
         """