feat(api): 补齐资源分发服务入口

新增 bat-api 资源 bootstrap/分发 HTTP 服务、RPC release 发现、CDN path 分发、launcher 资源引导兼容、控制面中间件、OpenAPI 和 systemd 模板。

同步 Go 边界文档，明确 Rust bat 是资源生产者和同步运维入口，Go bat-api 是只读 bootstrap/分发服务，试验 Go CLI 产物为 bin/bat-go。

验证：未运行新命令；本轮已按要求停止重复构建/测试。

api/openapi/README.md

@@ -0,0 +1,9 @@
+# bat-api OpenAPI
+
+`bat-api.yaml` describes the current resource bootstrap / read-only distribution
+HTTP surface. The running service also exposes the same contract at
+`GET /openapi.yaml`.
+
+This contract covers resource bootstrap, launcher resource compatibility,
+server-info rewrite, CDN-shaped resource bytes, auth schemes, and the reserved
+admin panel entry. It does not describe a full game business API.