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

docs/API.md

@@ -1,5 +1,7 @@
 # MC Report System API 文档
 
+> 面向**机器人 / 第三方系统**的免登录 API(鉴权方式、请求示例、子服定位语法)见独立文档 **[EXTERNAL-API.md](EXTERNAL-API.md)**。
+
 Base URL: `http://<host>:3100/api`
 
 ## 通用约定