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。

tests/test_media_ffmpeg.py

@@ -0,0 +1,147 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2025 relakkes@gmail.com
+#
+# This file is part of MediaCrawler project.
+# Repository: https://github.com/NanmiCoder/MediaCrawler/blob/main/tests/test_media_ffmpeg.py
+# GitHub: https://github.com/NanmiCoder
+# Licensed under NON-COMMERCIAL LEARNING LICENSE 1.1
+#
+
+"""ffmpeg 音视频合流测试。
+
+本机没装 ffmpeg 时整体跳过（不影响其他测试）。素材用 ffmpeg 内置的 lavfi 源生成，
+不依赖任何外部文件。
+"""
+
+from __future__ import annotations
+
+import asyncio
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+from media_downloader import merge_audio_video
+from media_downloader.ffmpeg import ffmpeg_path, is_available
+from media_downloader.types import MediaDownloadError
+
+pytestmark = pytest.mark.skipif(not is_available(), reason="本机未安装 ffmpeg")
+
+
+def _ffprobe_path() -> str:
+    probe = shutil.which("ffprobe")
+    if not probe:
+        pytest.skip("本机未安装 ffprobe")
+    return probe
+
+
+def _make_video(path: Path, seconds: int = 1) -> None:
+    # 显式 -f mp4：.m4s 不是 ffmpeg 能自动识别的容器后缀，需模拟 DASH 分段的真实形态
+    subprocess.run(
+        [
+            ffmpeg_path(), "-hide_banner", "-loglevel", "error", "-y",
+            "-f", "lavfi", "-i", f"color=c=red:s=320x240:d={seconds}",
+            "-c:v", "mpeg4", "-pix_fmt", "yuv420p", "-f", "mp4", str(path),
+        ],
+        check=True,
+        capture_output=True,
+    )
+
+
+def _make_audio(path: Path, seconds: int = 1) -> None:
+    subprocess.run(
+        [
+            ffmpeg_path(), "-hide_banner", "-loglevel", "error", "-y",
+            "-f", "lavfi", "-i", f"sine=f=440:d={seconds}",
+            "-c:a", "aac", "-f", "mp4", str(path),
+        ],
+        check=True,
+        capture_output=True,
+    )
+
+
+def _stream_types(path: Path) -> list[str]:
+    result = subprocess.run(
+        [
+            _ffprobe_path(), "-v", "error",
+            "-show_entries", "stream=codec_type",
+            "-of", "csv=p=0", str(path),
+        ],
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    return [line.strip() for line in result.stdout.splitlines() if line.strip()]
+
+
+@pytest.mark.asyncio
+async def test_merge_produces_playable_file_with_both_streams(tmp_path):
+    video = tmp_path / "video.m4s"
+    audio = tmp_path / "audio.m4s"
+    merged = tmp_path / "merged.mp4"
+    _make_video(video)
+    _make_audio(audio)
+
+    await merge_audio_video(video, audio, merged)
+
+    assert merged.exists()
+    assert merged.stat().st_size > 0
+    assert _stream_types(merged) == ["video", "audio"]
+
+
+@pytest.mark.asyncio
+async def test_merge_rejects_corrupt_input(tmp_path):
+    video = tmp_path / "video.m4s"
+    bad_audio = tmp_path / "audio.m4s"
+    merged = tmp_path / "merged.mp4"
+    _make_video(video)
+    bad_audio.write_bytes(b"this is definitely not an audio stream")
+
+    with pytest.raises(MediaDownloadError):
+        await merge_audio_video(video, bad_audio, merged)
+
+
+@pytest.mark.asyncio
+async def test_merge_kills_subprocess_when_cancelled(tmp_path, monkeypatch):
+    """任务被取消（Ctrl-C / abort）时不能留下孤儿 ffmpeg 进程"""
+    import media_downloader.ffmpeg as ffmpeg_module
+
+    # 必须用 exec：否则 sh 会另起一个 sleep 子进程，它继承着 stderr 管道，
+    # 即便 kill 掉 sh，管道清理仍会阻塞到 sleep 自然结束
+    fake_ffmpeg = tmp_path / "fake_ffmpeg.sh"
+    fake_ffmpeg.write_text("#!/bin/sh\nexec sleep 60\n")
+    fake_ffmpeg.chmod(0o755)
+    monkeypatch.setattr(ffmpeg_module, "ffmpeg_path", lambda: str(fake_ffmpeg))
+
+    task = asyncio.create_task(
+        ffmpeg_module.merge_audio_video(
+            tmp_path / "v.m4s", tmp_path / "a.m4s", tmp_path / "o.mp4", timeout=60
+        )
+    )
+    await asyncio.sleep(0.4)
+    task.cancel()
+    with pytest.raises(asyncio.CancelledError):
+        await task
+    await asyncio.sleep(0.4)
+
+    probe = subprocess.run(["pgrep", "-f", str(fake_ffmpeg)], capture_output=True, text=True)
+    assert probe.returncode != 0, f"取消后子进程仍在运行: {probe.stdout.strip()}"
+
+
+@pytest.mark.asyncio
+async def test_merge_reports_missing_ffmpeg(tmp_path, monkeypatch):
+    import media_downloader.ffmpeg as ffmpeg_module
+
+    monkeypatch.setattr(ffmpeg_module, "ffmpeg_path", lambda: None)
+    video = tmp_path / "video.m4s"
+    audio = tmp_path / "audio.m4s"
+    video.write_bytes(b"x")
+    audio.write_bytes(b"y")
+
+    with pytest.raises(MediaDownloadError, match="ffmpeg"):
+        await merge_audio_video(video, audio, tmp_path / "out.mp4")
+
+
+def test_is_available_matches_executable():
+    assert is_available() is bool(ffmpeg_path())