docs: standalone external API doc for bots/third-party

- docs/EXTERNAL-API.md: auth matrix (ID+Secret / legacy key / JWT),
  Python+Node quickstart, server alias syntax, 9 endpoints with
  request/response examples, status flow, error codes, troubleshooting,
  typical scenarios (QQ bot ticket flow, plugin ban sync)
- README + API.md link to the standalone doc
- verified: 24 checks, endpoints cross-checked doc vs code

README.md

@@ -22,7 +22,8 @@
 
 ## API 文档
 
-完整接口文档见 **[docs/API.md](docs/API.md)**（认证/工单/封禁/用户/设置/通知/投票/更新/导出/外部插件 API）。
+- **完整接口文档**:[docs/API.md](docs/API.md)(认证/工单/封禁/用户/设置/通知/投票/更新/导出/外部插件 API)
+- **第三方外部 API(机器人/插件)**:[docs/EXTERNAL-API.md](docs/EXTERNAL-API.md)(ID+Secret 鉴权、提交工单、进度追踪、封禁拉取、按子服拉数据)
 
 ## 快速开始