refactor: 移除独立 hub/proxy/executor/gateway crate，统一为 gateway tunnel 架构

- 删除 aether-hub、aether-proxy 独立项目及其 Dockerfile/配置
- 删除 crates/aether-executor 和 crates/aether-gateway 全部模块
- 新增 apps/ 目录作为应用入口
- 将 hub 概念重构为 gateway tunnel transport
- 将 executor 重构为 execution runtime
- 新增 tunnel.rs 合约定义和 testkit tunnel/execution_runtime 模块
- 更新 Python 服务层和测试适配新架构命名

apps/aether-gateway/src/api/backend/internal.rs

@@ -0,0 +1,19 @@
+use axum::routing::{any, get, post};
+use axum::Router;
+
+use crate::gateway::{
+    proxy_request, proxy_tunnel, relay_request, AppState, PROXY_TUNNEL_PATH, TUNNEL_HEARTBEAT_PATH,
+    TUNNEL_NODE_STATUS_PATH, TUNNEL_RELAY_PATH_PATTERN,
+};
+
+pub(crate) fn mount_internal_routes(router: Router<AppState>) -> Router<AppState> {
+    router
+        .route(
+            "/api/internal/gateway/{*legacy_gateway_path}",
+            any(proxy_request),
+        )
+        .route(PROXY_TUNNEL_PATH, get(proxy_tunnel))
+        .route(TUNNEL_HEARTBEAT_PATH, post(proxy_request))
+        .route(TUNNEL_NODE_STATUS_PATH, post(proxy_request))
+        .route(TUNNEL_RELAY_PATH_PATTERN, post(relay_request))
+}