feat(fm): let each web user link their own NetEase account for personal FM (#164)

With several people sharing one bot, personal FM always followed the one
account the bot was logged in with. Each signed-in (non-guest) web user
can now scan a QR code under Settings → 账户 to link their own NetEase
account; FM they start from the WebUI then comes from their account.

- user_music_cookies table (per user + platform, dropped with the user).
- NeteaseProvider.pollQrLogin returns the cookie without storing it, so
  a personal login can never replace the bot's shared account;
  checkQrCodeStatus is now built on it. withCookie gives a view bound to
  another account.
- /api/me/music/netease: status / qrcode / qrcode/status / unlink, acting
  only on req.user. The cookie never leaves the server.
- POST /api/player/:botId/fm uses the caller's linked account for
  NetEase. Songs still resolve through the shared provider when played.

TeamSpeak chat !fm keeps using the shared account: chat users are not
tied to web accounts.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -42,7 +42,7 @@
 - **完整播放控制** — 播放/暂停/上一首/下一首/进度跳转/音量调节
 - **四种播放模式** — 顺序播放/循环播放/随机播放/随机循环
 - **实时歌词同步** — 歌词滚动显示，支持翻译歌词，服务端帧计数精确同步
-- **歌单管理** — 推荐歌单/我的歌单/每日推荐/私人FM，点击播放全部；私人 FM 支持网易云、**QQ 音乐雷达推荐**（`!fm -q`）与**酷狗私人电台**（`!fm -k`）。网易云、QQ、酷狗均提供登录后的推荐歌单 / 每日推荐 / 我的歌单
+- **歌单管理** — 推荐歌单/我的歌单/每日推荐/私人FM，点击播放全部；私人 FM 支持网易云、**QQ 音乐雷达推荐**（`!fm -q`）与**酷狗私人电台**（`!fm -k`）。网易云、QQ、酷狗均提供登录后的推荐歌单 / 每日推荐 / 我的歌单。多人共用时，每个网页端用户可在 **设置 → 账户** 扫码绑定**自己的网易云账号**，之后他在网页端开启的网易云私人 FM 按他自己的口味推荐（未绑定则用机器人的共享账号；TS 聊天里的 `!fm` 仍用共享账号）
 - **音质选择** — 标准(128k) / 较高(192k) / 极高(320k) / 无损(FLAC) / Hi-Res / 超清母带
 - **B站视频音频提取** — 搜索B站视频，自动提取DASH最高码率音频流播放
 - **B站热门推荐** — 首页展示B站热门视频和个性化推荐（登录后更准确）