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/kuaishou/core.py

@@ -35,6 +35,7 @@ from playwright.async_api import (
 
 import config
 from base.base_crawler import AbstractCrawler
+from media_downloader import MediaDownloader
 from model.m_kuaishou import VideoUrlInfo, CreatorUrlInfo
 from proxy.proxy_ip_pool import IpInfoModel, create_ip_pool
 from store import kuaishou as kuaishou_store
@@ -42,6 +43,7 @@ from tools import utils
 from tools.cdp_browser import CDPBrowserManager
 from var import comment_tasks_var, crawler_type_var, source_keyword_var
 
+from . import media as kuaishou_media
 from .client import KuaiShouClient
 from .exception import DataFetchError
 from .help import (
@@ -64,6 +66,7 @@ class KuaishouCrawler(AbstractCrawler):
         self.user_agent = utils.get_user_agent()
         self.cdp_manager = None
         self.ip_proxy_pool = None  # Proxy IP pool, used for automatic proxy refresh
+        self._media_downloader: Optional[MediaDownloader] = None
 
     async def start(self):
         playwright_proxy_format, httpx_proxy_format = None, None
@@ -177,6 +180,7 @@ class KuaishouCrawler(AbstractCrawler):
                 for video_detail in videos_res.get("feeds", []):
                     video_id_list.append(video_detail.get("photo", {}).get("id"))
                     await kuaishou_store.update_kuaishou_video(video_item=video_detail)
+                    await self.download_media(video_item=video_detail)
 
                 utils.logger.info(
                     f"[KuaishouCrawler.search] keyword: {keyword}, page: {page}, got {len(video_id_list)} videos"
@@ -215,6 +219,7 @@ class KuaishouCrawler(AbstractCrawler):
         for video_detail in video_details:
             if video_detail is not None:
                 await kuaishou_store.update_kuaishou_video(video_detail)
+                await self.download_media(video_item=video_detail)
         await self.batch_get_video_comments(video_ids)
 
     async def get_video_info_task(
@@ -464,6 +469,49 @@ class KuaishouCrawler(AbstractCrawler):
         for video_detail in video_details:
             if video_detail is not None:
                 await kuaishou_store.update_kuaishou_video(video_detail)
+                await self.download_media(video_item=video_detail)
+
+    async def download_media(self, video_item: Dict):
+        """下载快手作品的媒体资源（封面 + 视频）
+
+        Args:
+            video_item (Dict): 快手作品详情（结构为 {"photo": {...}, "author": {...}}）
+        """
+        if not config.ENABLE_GET_MEDIA:
+            return
+        try:
+            items = kuaishou_media.build_media_items(video_item)
+            if items:
+                await self._get_media_downloader().download_all(items)
+        except Exception as exc:
+            # 媒体下载是旁路能力，解析异常/网络异常都不能中断爬取主流程
+            utils.logger.error(f"[KuaishouCrawler.download_media] 媒体下载异常: {exc}")
+
+    def _get_media_downloader(self) -> MediaDownloader:
+        """惰性创建媒体下载器，并同步最新的代理与 UA（代理池是就地刷新的）"""
+        if self._media_downloader is None:
+            self._media_downloader = MediaDownloader(
+                platform="ks",
+                proxy=getattr(self.ks_client, "proxy", None),
+                extra_headers=self._media_headers(),
+            )
+        else:
+            self._media_downloader.update_credentials(
+                proxy=getattr(self.ks_client, "proxy", None),
+                extra_headers=self._media_headers(),
+            )
+        return self._media_downloader
+
+    def _media_headers(self) -> Dict:
+        """媒体请求头：平台 Referer（防盗链）+ UA。
+
+        client.headers 里含 Cookie，不能整体透传到 CDN 请求上，因此只取 UA。
+        """
+        headers = {"Referer": "https://www.kuaishou.com/"}
+        client_headers = getattr(self.ks_client, "headers", {}) or {}
+        if client_headers.get("User-Agent"):
+            headers["User-Agent"] = client_headers["User-Agent"]
+        return headers
 
     async def close(self):
         """Close browser context"""